Registry is mess. How can I clean registry in windows 10?

Your caution is right, Windows registry is very critical, changing it badly will make the system out of big problems, in fact, better than changing the registry directly is to fix the system file errors first, and then delete the files corresponding to those outdated registry entries, which is more effective and safer.

How to safely clean registry in Windows 10 

Step 1: Clean up registry-associated redundant files with Disk Cleanup

• Press Win + R and type cleanmgr to open the Disk Cleanup tool.
• Select the C drive and click Clean up system files.
• Check all the file categories and focus on Windows Update to clean up previous Windows installations.
• Click OK to delete, free up space while clearing the corresponding invalid registry data.

Step 2: Repair Associated System Files with System File Checker

• Search for cmd, right-click Command Prompt, and select Run as administrator.
• Enter the command sfc /scannow and press Enter.
• Wait for the scanning to complete, the tool will automatically detect and repair damaged Windows system files.

Step 3: Repair Windows image with DISM

• Continue to operate in the administrator command prompt window.

Type the command: DISM /Online /Cleanup-Image /RestoreHealth and press Enter.

• Wait for the command to complete and finish repairing the Windows image.

I need to tell you to avoid high-risk operations: 

1. It is not recommended to delete registry entries manually with Regedit, without clear knowledge of the deletion, easy to cause system crashes or software failure.
2. Choose third-party registry cleaner tools carefully, most of them delete redundant entries that may still be useful and have limited performance improvement on modern SSD computers, but may cause new problems.