If you mean that you want to change the username folder then you can't. You will need to create a new user with the required name then transfer anything you want to keep to that user. click here for info on how to do it.
Give the name of the extension you are trying to rename but you should be aware that the file will be unusable with a program that created or uses the file when you have done it. I have knowingly changed an exe file that I was suspicious of (while I investigated it's origin and use)to exb but I know what I am doing and will change it back if I need to. i.e if you change a jpg file to a doc file, word will not open it.