How do I change the avatar that is linked to my web account/HUD?

Step 1 - Unlink existing avatar

From the website menu, select Edit Profile (from the Profile menu) and from there you will see the details of the currently linked avatar.  Click on the Unlink Avatar... button then the Confirm Unlink button.

Step 2 - Link new avatar

You can now go back to the edit profile page and from here you will see the link code to use. Log into OpenSim using the new avatar you wish to link and head to one of the terminals on any of the VivoSim regions (e.g hg.vivosim.net:8002:mintor) and touch the VivoSim terminal to start the linking process.