To start,
Go to Member Settings. You will see two options
Assign Members to Rosters
Select this if you want to Unassign/Assign to user from a team
Remove Members from League
Select this if you want to remove a player from the league
If you want to unassign a user, you will see a screen like this.
Press UNASSIGN to remove the owner from a team
Invite the new owner
Go back to Member Settings
Hit the Plus icon next to the team you want to assign
Removing a owner will take them out of the entire league. This will not impact any players or draft picks that are on the actual team.
Unassigning a owner will not remove them from the league.