As the head, you can promote any guild member to be co-head and then to be head. To do this, tap on the guild member management button.
If the head leaves the guild or doesn't play the game for more than 30 days, their place will be taken by the co-head of the highest level, provided they have played the game within the last 3 days. If there are no co-heads in the guild, the new head will be chosen from among the other guild members.
When leadership of the guild is handed over, you lose your status as guild head. There can never be more than one head at a time in the game.