how do I close unity - and release resources

I have a folder (named cs 426) which inside that folder contains the folder of my project. I want to change the name of the "cs 426" folder but I can't because it says it is still in use (it only contains unity related files and folders). However, Unity is "closed". How do I truly close Unity so that it let's go of the files? That way I can change my folders around.