Use the pointed end of a spudger to depress the metal tab on the power button cable's board connector.

With the metal tab depressed, use a pair of tweezers to grip the plastic pull tab and slide the cable out of its socket.

Pull only on the pull tab, not the cable itself.

Don't pull on the cable without depressing the metal tab, otherwise you risk damaging either the cable or the connector.

During reassembly, the cable should lightly "snap" into place when inserted.