1. Hover over your account name in the top right
Select "Manage License Keys" to head to your user profile.
2. Find your license key you wish to revoke from an assignee
Once you've found the license, click the grey cog icon to enter the license key.
3. Click the red cross icon to remove the user
Once the user has been removed, you can either activate the license for yourself or re-assign the license to another user, see Assigning a License or Activating a License.