Because you cannot be a part of more than one neighborhood, you will need to first leave the neighborhood you are currently in, in order to join another one.
To leave your neighborhood, you will need to head over to your neighborhood house, tap on the button “Leave” and confirm. Notice that every time you leave a neighborhood there is a cool down time before you can join another one. After this cool down period, you will be able to join the neighborhood you want to be a part of. But then again, why not start your own neighborhood?