As it turns out, this is more simple than you would usually figure. All you do is the following:

  • Go to Site Actions/Site Permissions.
  • Then, check the box which is to the left of the group you need to edit.
  • In the Ribbon, click the ‘Edit User Permissions’ button (also works for individual users).
  • Adjust the permissions as needed and click the OK button.

Voila! You’re done.