You'll have to change the email address for your Apple ID first.
Sign out in Settings - iTunes & App Store. If you use the same ID for iCloud as well, sign out there too (disable Find my iPad first).
Go to this site:
https://appleid.apple.com
Select "Manage Apple ID" and sign in.
You'll find the procedure for changing the Apple ID here:
http://www.ipadforums.net/threads/need-help-changing-ipad-details.128123/#post-1102373
After verifying the ID, you can sign in in Settings - iTunes & App Store, as well as in Settings - iCloud.
Hope that helps.