To edit your PC in a friend's campaign, your friend must first assign you as the player of that character. While logged in, navigate to your character page and click the "Claim" button to send your GM an email.
Once you're assigned as the player, you can edit your character with the "Edit" or "Edit Connections" buttons.
If you are a Power Player, then you can edit any page in your friend's campaign, not just your character. So you can help with keeping logs, creating NPC's mentioned in your PC's backstory, etc.