To begin, click on the SideDrawer management button (the one with the 3 lines) in the top right of your screen.
Here we see that there are 2 current collaborators to this SideDrawer, Alexander and Gaston. Next, choose the collaborator that you'd like to change permissions too.
You can see what Alexander's current level of permission is by the icon to the right of his name. In this case, Alexander has editor permission.
To change Alexander's permission, click the little down arrow and a box will open for you to make changes.
From the drop box, simply click the level of access you'd like Alexander to have.
You can see that the permission icon to the right of Alexander's name has now changed to viewer. Simply click "Update" and you're finished!