How can I change the username of the account owner?

Account owner's email id cannot be changed directly, the ownership needs to transferred to another admin user temporarily and only then can you proceed with the modification in the username.

1. Click on Menu > Manage Users
2. Transfer the ownership to another admin user by clicking on the Customize > Change Owner 
3. Now change the login username for the actual account owner by clicking on your user in the Manage Users page > More > Change Username
4. Transfer the ownership back to the original owner.