Once you've got it working the first time, and thus have the correct connection between AB and Gravatar, it's easy-peasy thereafter.
Load up a new avatar in Gravatar, give it a few minutes, then go to edit your profile in AB; if your new avatar isn't displayed there then hit ctrl-F5;. As soon as it is displayed save your profile, and when you are on any page where your old avatar is still shown hit ctrl-F5 again.
Eventually when all the various caching has been cleared your new one will be on display.
It is easy - changed mine again just to demonstrate, it took just over 5 mins from uploading a new avatar to Gravatar until AB first noticed it in my user profile.