You can only be a member of one team at a time. If the team you want to switch to is an open team, you'll be prompted to confirm the team change after tapping Join. If the team is closed, you need to leave your current team first.
To leave the team, tap the About button in the My team tab and tap Leave.
Be careful: if you change teams frequently, you'll have to wait a while before you can request lives from your new team members.