Best way to speed up my windows 10 pc as running very slow

A few ways to fix that:

• Upgrade to Windows 11
• Add RAM
• Remove unused apps for extra disk space
• Use PC Manager (IF your Windows 10 Version is 19042.0 or above) and click "speed up PC"
• DO NOT USE UNOFFICIAL (Unofficial: refers to Microsoft-recommended) THIRD-PARTY "CLEANERS"
• Attempt to free more space by clearing caches and Windows Update Files (those files cause some disk space to be taken but unshown in any folder)

A thing I know is that it might have been caused by Windows Update, as said by you. Check Settings -> System -> Storage -> Cleanup Recommendations and find "Update Files". Click to remove them.

If none works, consider (you won't have to, but consider that) replacing it, as sometimes your computer's age might affect your computer's speed.

Tell me one thing if none works: How old is your PC?

If it's old, probably just replace it.