Q: How can I change my EMail adress on iCloud?

I just purchased a new iMac 21.5" and upgraded it to Yosemity O.S.

I set it up using one eMail adress (hotmail) and password. I have, now changed it to a different eMail adress (gmail) and password. But when I try to log in to iCloud it still shows my old eMail adress and it won't accept my new password. I believe that I need to change the email adress but how can I do this?

Please help.