Contact support with Guild Management selected if your leader is banned and you need to change leaders.
You're welcome to appeal a ban for your Guild Leader, however, technical support can only discuss bans with the player that was banned and won't be able to comment on the ban case.
Replacing the Guild leader
If your Guild master or leader is banned and you need someone else to take over you can contact support here and request yourself or the next highest rank in command be placed into command. Note that most bans are permanent and your Leader may not be unbanned. Alternatively, you can wait until the leader has become inactive for at least 21 + Days where they can then be kicked by the next highest rank in command.