You can only change another user's access rights by being Owner level yourself.
To change another user's owner rights, navigate to the Admin section at the top of the screen, then click Team on the left-hand side. From there you can search for team members, and once you've found the correct person click the 3 vertical dots on the right-hand side of the white box with their name in it.
From there you will see their current access rights - Owner, Member or Restricted.
When you click into their current access rights, you will then see all the other potential access rights appear and you can select from there.
You can also remove a team user by clicking 'Delete' in the drop-down list. Note that the said user will lose access to your team account.