How do I change someone's role or remove them from my team?

Do both from the People list in Settings → Team. Pick a new role from the menu next to their name, or click Remove and confirm.

Someone you remove is signed out everywhere at once, and their seat frees up straight away. A pending invite can be withdrawn the same way with Withdraw. Owners and admins can manage everyone except the owner, and a teammate can click Leave to take themselves off.