If your Guild Master does not log in to the game for more than 30 days in a row, their title is automatically transferred to an active player with the highest position after the 30-day timer has run out.
If the Guild Master logs in, the 30-day timer resets. Another way to transfer the title is to contact that player directly and ask them to manually transfer it to someone else.
Here you can find instructions on how the Master can transfer their title to another player.