How can I upgrade Windows 10 to Windows 11 25H2 without data loss?

Yes. You can safely upgrade to Windows 11 25H2 from Windows 10 for free. Make sure the processor supports SSE 4.2. 

Hello! Given your specifications and goal to upgrade Windows 10 to Windows 11 25H2 without data loss via registry hacks or scripts, Step-by-Step guide to attempt an In-Place upgrade via Registry Hacks:
1. Backup Your System
Use Windows Backup or create a full system image.

2. Prepare Your System
Fully update Windows 10.
Download the Windows 11 25H2 ISO from Microsoft.

3. Modify Registry to Bypass Hardware Checks
Open Registry Editor, Press Win + R, type regedit, press Enter.
Navigate to the key: HKEY_LOCAL_MACHINE\SYSTEM\Setup\MoSetup
Create or modify values: AllowUpgradesWithUnsupportedTPMOrCPU (DWORD): Set to 1  
Right-click in right pane > New > DWORD (32-bit Value)
Name: AllowUpgradesWithUnsupportedTPMOrCPU
Data: 1
AllowUpgradesWithUnsupportedSecureBoot (DWORD): Set to 1 (if present)
Optional:
Create or set SkipTPMCheck and SkipSecureBootCheck to 1 in the same location.

4. Initiate the Upgrade
Mount the Windows 11 ISO:  
Double-click the ISO file to mount it as a virtual drive.
Run setup.exe:  
Navigate to the mounted ISO and launch setup.exe.
Follow the prompts:  
Choose "Keep personal files and apps" for an in-place upgrade.
Allow to completely upgrade Windows 10 to Windows 11 25H2.

About your question of upgrading Windows 10 to Windows 1125H2. Older hardware, like your Intel Core i7-6800K and GTX 1070, may encounter driver compatibility issues, potentially leading to stability problems, reduced performance, or device malfunctions. Some older or less common programs might not be fully compatible with Windows 11, which could cause crashes or require updates.

Windows upgrades can sometimes introduce bugs or stability issues, especially on systems that are a few generations old or have complex configurations. Some users experience system crashes, freezes, or unexpected behavior after upgrading. 

While upgrading Windows 10 to Windows 1125H2 without data loss is possible and often successful, these potential disadvantages highlight the importance of proper preparation. If your hardware and software are compatible and you take backups, the risks are minimized. However, be aware of possible stability, performance, and compatibility issues, especially given your hardware's age relative to Windows 11’s system requirements.

You can't directly upgrade Windows 10 to Windows 11 25H2.  Microsoft does not offer a free direct upgrade path to a specific feature update. Windows 11 25H2 is a feature update to Windows 11, not an entirely separate version. You need to first upgrade to Windows 11 (which is a one-time free upgrade from Windows 10) and then receive the 25H2 update through normal Windows updates.

The steps are:
1. Keep your data, programs, and settings:  Windows 11, when upgraded from Windows 10, should preserve your files, programs, and settings.  However, it's always a good idea to:

• Back up your data: This is essential, regardless of whether you're upgrading. Use a cloud service, external drive, or another method to back up important files.
• Consider a system image: This creates a backup of your entire system, which can be incredibly helpful if something goes wrong during the upgrade or later.

2. Update to Windows 10 to Windows 11 25H2:  Once you're running Windows 11, Windows Update will automatically offer you the latest feature updates.  You should not have to do anything special.  Just let Windows Update run in the background.

There are several ways to upgrade Windows 10 to Windows 11 25H2, one of which I recommend is more controllable and good for when the installation assistant fails to upgrade properly.

1. Open the official Windows 11 download page and locate the Create Windows 11 installation media section.
2. Download the Media Creation Tool and run it.
3. Follow the steps prompted by the tool to step through the creation of a bootable USB drive.
4. With Windows 10 running normally, insert the created bootable USB drive into your computer.
5. Open File Explorer, locate and enter the directory where the USB drive is located, and double click to run the setup.exe file.
6. When prompted to choose how to proceed, select Download and Install Update.
7. Read and accept the terms of the license, go to the Select what to keep screen, and check Keep personal files and applications.
8. Follow the on-screen prompts to complete the remaining operations and wait for the upgrade process to finish.

Backing up your personal files before upgrading is a crucial step to minimize data loss due to unexpected circumstances. According to the method I gave you, when upgrading to Windows 11 25H2, you can not only retain the original files and applications, but also experience the new version's functions. The whole process is safe and efficient, allowing you to easily start using and experiencing the new version.

Upgrading from Windows 10 to Windows 11 25H2 is a straightforward process, but it requires your computer to meet specific hardware requirements. The most common obstacles are TPM 2.0 and CPU requirements.

How to upgrade Windows 10 to Windows 11 25H2

Step 1：Ensure Windows 10 is up to date

• Open Settings → Go to Update  Security → Select Windows Update and install all available updates.
•  Note: Your system must be upgraded to at least Windows 10 version 22H2 to proceed.

Step 2. Enable TPM and Secure Boot

These features are often disabled by default in BIOS and require manual activation:

• Restart your computer. During startup, quickly press F2, Del, or F10 to enter the BIOS/UEFI settings interface.
• Locate the TPM and Secure Boot options in the settings and set both to Enabled.
  Save settings and exit BIOS. Your computer will automatically restart.

Step 3: Initiate the Upgrade Process

• Open the official Microsoft Windows 11 download page.
• Under the Windows 11 Installation Assistant section, click Download Now.
• Locate the downloaded Installation Assistant file and run it, following the on screen prompts step by step.
• The Installation Assistant will automatically complete the entire upgrade process, preserving your personal files and installed applications throughout.

I must emphasize: Enter the BIOS/UEFI setup interface and locate the TPM setting
(displayed as PTT for Intel CPUs, and possibly as PSP for AMD CPUs).