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

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.