its probably been asked before

[bubonicone]

I'm having problems with an extension and a ruleset
the extension creator said to uninstall everything and reinstall again
where are the files with steam version i need to remove to uninstall it completely

[damned]

Extensions cannot effect the installation itself.
Do not uninstall the application.
Probably what you want to do is remove:
CoreRPG.pak
The ruleset that you are playing - eg 5E.pak
The extension.ext you are using

Then rerun the updater and copy the extension back in.

Other things that are likely... it could be another extension conflicting with the one you are using.

Anyway - you can find your files by launching FG and on the splash screen there is an Explorer Icon - this will open to the Fantasy Grounds data folder.