Roles can be edited via the  Assign Role   and for the current folder also via  File    Share    Assign Role   , action.

This brings up a form with a list of all users which have access to the object. For each user the currently available roles are shown and one of the roles can then be selected by clicking the respective toggle button. The role which is currently assigned to the respective user is pre-selected, i.e., if the role of a user shall not be modified, no action is required for the respective user.