The Edit Document Library and List Permission sceens allows you to easily modify permissions on SharePoint Document libraries and lists in Office 365.

EditDocumentLibraryPermissions

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To use the Edit Document Library Permissions, you must first select the Site Collection, Site Name, and Document Library name.  Clicking the Get Existing Permissions button will retrieve the existing permissions on that Document Library.

Once the existing permissions are displayed you can either Add/Remove users or if document library has custom permissions set, you can reset the role inheritance so that the existing permissions set at higher levels are replicated down.