If you have connected to a source repository service with the wrong account, you will have to go to your user settings in the source hosting service and revoke the OAuth token manually. Then you will be able to re-connect to another account in App Center.
Keep in mind that if you connect to a new account, you will lose all the build configuration and previous builds for ALL the apps configured with the previous account.
In order to connect to another GitHub account to build your apps, first step is to revoke the OAuth token: go to https://github.com/settings/profile > Authorized Applications > Revoke App Center token. Then, go back to App Center in the build tab, reconnect to GitHub and go through the OAuth flow again. Make sure that before you re-do the OAuth flow in App Center, you log out in that browser from GitHub first.