Both are done from your existing entry, so you do not lose your place in the event.
Leaving a team
Make sure you are logged in to Race Space.
Select My Entries from the main menu.
Find your ticket on the My Entries tab.
Select Edit Entry from the Entry Actions menu next to it.
Under Team Details, select Leave Team.
Switching to a different team
Follow the same steps, but instead of stopping at Leave Team, type the name of the team you would like to join and then select Update Entry.
Starting a new team instead
Type your new team name into the same field. Be careful here: if the name matches a team that already exists for this event, you will join that team rather than create your own. Pick something distinctive.
If you are the Team Captain, you have to hand the role to another team member before you can leave.