For windows machine
you can manage windows credential for doing this
go to search button then write
manage windows credential
then you will find a button double click it
then remove your current windows credential
after that when you are using git again git authentication will be asked
then you will enter your new github id
Next you need to edit .gitconfig file
file path will be like
C:\Users\Imu
here imu is my user name
it will be different in yours
Now edit .gitconfig file using text editor like nodepad++ or sublime text with ur current id information
hoping it helps