You can give a collaborator a higher access level at a subfolder and then a lower permission to that subfolder's Parent folder. You must first invite the collaborator to the subfolder, then invite the collaborator to the parent folder.  For example, you could invite John to the subfolder "Marketing folder" as an Co-Owner, then invite him to the Parent folder as a Viewer. You must invite the user in this order to achieve this goal, otherwise, if you try changing the access level for any subfolder would cause the individual's access level also to change for the parent folder and all its subfolders. The system displays a message stating this information when changing someone's access level in a subfolder instead of in the parent folder. 

For more details, please take a look at this article: https://support.box.com/hc/en-us/articles/360043697254-Understanding-Folder-Permissions  

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