Hello GEeeek, and welcome to Seven Forums.
You could use the tutorial below to properly change the name of your user profile and it's C:\Users\(user-name) folder. However, it may be easier to just create a new administrator account with the name you want, copy any files (documents, music, shortcuts, etc...) from the old account to the new one, log on to the new account, then just delete the old account when ready. User Profile Folder - Change User Account Folder Name
Hope this helps,