Change a User or Group’s Permissions on a File or List Item in SharePoint

To change a permission on a file or list item simply naivgate to the file or list item, hover the mouse over it, click the dropdown menu and select Manage Permissions:

This will take you to the Manage Permissions page where you can select a user or group and select Edit User Permissions from the Actions menu. This will direct you to the Edit Permissions page where you can assign or remove permissions.