To do this, go to your avatar page where you will see this button to the left of your current avatar:
Clicking on it will save your current avatar, and it will appear right below the button you just pushed. Now you may upload your new avatar to the site.
One at a time, please!If, for some reason, your avatar upload doesn't "take" (or if an avatar is denied) your complete avatar library will not show. However, the library remains intact, and if you upload another, it will show up again. Your avatars are not lost.
Remember that there may be cache shift-reload delays when using this feature. If you think you have switched but still see your original avatar, always try a shift-reload (Shift + F5 in IE). Also keep in mind that it takes some time for all servers to be updated. It may be necessary to clear your browser's cache in order to see your updated avatar when using the library feature. For more info on this, see this thread.
Remember also that you must have a premium membership to use this feature.