Setting different level of access for a user within a structure with subfolders

In a structure with a Root folder with several subfolders, some of them also having subfolders (that's 3 levels), how can I change the level of access for 1 specific user for specific subfolders? Example: the user should have "previewer uploader" role for the Root folder and the first level of subfolders attached to the Root, but should have an "editor" role in a specific sub-subfolder? Each time we want to change the role of the user, it says that the role will change also at Root level as it is the parent folder.