The folders in C:\Users are correlated to the username attached to a user and special folders such as Public and Default. The public and default folders are very important and should not change. The User folders are your profile. Windows expect the folder to be there in order to load it, or you will get errors such as "Windows tried to load your profile but was unable and signed you in with a temporary profile. Any changes you make will not be saved."
The only way to change this folder is by actually changing the username first, then change all the places where the username is stored, such as in the registry at many places, Computer Management (Users and groups) and the C:\users folder.
The trick becomes that you have to do this from a different user in order to make sure the folder is not locked which makes this a really hard change to do and frankly is outside of the scope of SuperUser. I bet there are guides available on how to do this somewhere, but the far easier approach would be to create a new user with the correct username, migrate your data to it (which can be done using a tool called Profile Wizard (not free, but the shareware version should suffice) and then remove the old profile.
If your computer is part of a domain, the trick is actually really easy.
First you ensure that you are using roaming profiles, such that your profile is stored on the server. Then you logout, on the server in Active Directory, you change the username, and the next time you login, all is changed and the old folder in C:\Users can be removed. Optionally you can disable roaming profiles if it was disabled before.
Do note that changing the username can still have repercussions. If you for example use Outlook, it will expect the data to be stored at a certain place and outlook may not start anymore. A new Outlook profile will fix that. Other programs may have similar problems.
From an IT professional perspective: if the user is very new, you can change the username easily without much problems, but if the user already uses it for at least a few days, you will consider strongly to not change it anymore and offer to create a new user with fresh settings instead and migrate the data to it.