nexus 7 2013 gmail has att email attached - how to change to gmail account

[sakqweasd]

when I first started my Nexus 7 it asked for an email address. I put in my at&t email address by mistake. Now I'm the gmail icon it keeps telling me that my ATT email address should sync. I was able to set up my gmail email account on the email icon on my Nexus. However even though I can use the email icon on my Nexus with my gmail account, I cannot access my contacts from there. I suspect that if I would have put in the proper Gmail account into Gmail icon on the Nexus I would be able to get my contacts. The problem is does not seem to be any way of changing the gmail address once it has been entered. Does anyone know how to change the gmail address on the Nexus 7 2013 to another account?