signing in Android market

[Android Question]

I have Samsung Galaxy mini s3 and I'm having difficulties signing in on android market. my friend accidentally typed his e-mail adress in the username field and it would be okay it allowed me to change it but now i can't acces android market at all because i can't change the username field i can only type in my password, pleasee someone help i cant download any apps because of it and it pops out every time i get on the internet
if you can help me, i'll be thankful. )

 

[D-U-R-X]

If you head to "settings=>apps=>Google Play Store" and then clear data, that should pop up a message stating the

All this app's data will be deleted permanently. This includes all files, settings, accounts, databases, etc.

I believe this should solve the issue