[1] The safest way is to let Windows do it
Go to Settings -> System -> Storage -> Temporary Files.
Select everything and click on Remove files.
[2] Use an external tool
Example: PatchCleaner.
[3] Delete old files from the temporary directory
This directory is usually located in: C:\Users\<YOUR_USERNAME>\AppData\Local.
Here you can safely delete months or years old files. If they're still in use, Windows will prevent you from deleting them.
If you're in doubt, skip what you were planning to do.
