If you'd like to join a new Alliance or switch from your current one, follow these steps:
Go to the game's Home screen.
Tap ALLIANCE.
Tap Alliance Search.
Find an Alliance using one of the following methods:
Search for an Alliance by name.
Browse the recommended Alliances list, which displays public Alliances that are open to join.
Select the Alliance you'd like to join.
Tap Join and confirm your choice.
If you're already a member of an Alliance, you'll be asked to confirm that you want to leave your current Alliance before joining the new one.