I use Git GNU bash, version 4.3.42(5)-release (x86_64-pc-msys) windows 8.1 pro. I gave it a username at the very beginning by typing the command: git config --global user.n
The suggestion made above was great, but i managed differently to solve the problem :
I went to the Git GUI, then Edit, Then options and there i've found different button to change the username. That was successful.