If a user needs to change gyms, either because they are moving to a new gym or they registered for the wrong gym, they can update their own profile. Coaches/owners can only edit users who are already in their gym.
After logging in, go to the users profile, then click Edit Account.
On the first tab, Profile Info, there is a "Gym" setting. Just delete the entry and start typing the new gym name, suggestions will pop up for you to select from. Then Save your profile.
The user may also want to change what Programs they are following. This can be done via the "Following" tab of the Edit Account screen; or you can click "Find Programs" in the left side navigation menu anytime.