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what do you want to edit, the actual grahic or the link to the graphic? If you want to edit the graphic then you can download the graphic and change what you want with GIMP or similar program and then upload it again.
If you want to edit which avatar appears for a particular user then it is simple to change the database in phpMyAdmin.
1) As with any change, take a backup of your database before you attempt the change.
2) select which avatar you want, find the avatar name that you want in the avatar table. Note the avatar_id.
3) Then change the avatar_id field in the avatar_user_link table for the user to match the one that you want for them.