To be able to remove a team member, just simply click 'Team' at the navigation bar then select 'Manage team'.
You'll see the list of your team members and from this page you'll be able to remove them from your team if you want to downgrade your subscription. Just look for (x) button to Remove user.
Please note that you are not able to downgrade your subscription once you haven't remove your team member. You should remove it first so you can select 1 seat at the Billing page to downgrade.