Email Address Change (with GUID screenshot)
If your email address is incorrectly entered and you have a GUID registered, you should create a ticket using your (new) e-mail address (make sure it's not in use already) and your account HID. You must provide proof that the GUID, and therefore account, belongs to you by making a screenshot of your GUID in-game as explained here.

IMPORTANT: If you do not have a GUID registered that will confirm your identity, you must make a post on the HelpDesk forum, so that your clanleader can confirm your identity. If you are the clanleader, then ask a trustee or two clanmembers to reply to the message. Your identity can not be confirmed by others in a HelpDesk Ticket.

Password Reset (with GUID screenshot)
You can request a password reset on the Lost Password page. If your account has not been used for a long period, then your identity may need to be confirmed by a clanleader or trustee. If there are no active clan members able to confirm your identity and you have a GUID registered, you should create a HelpDesk Ticket including an in-game GUID screenshot as explained here.

Duplicate GUID (with GUID screenshot)
If you cannot register your GUID because it is already registered with somebody else's account, please create a ticket including the HID or the link to the other account in the message (This link will show up in the error message you get). To prove it's actually your GUID, please attach an in-game GUID screenshot as explained here.