I want to connect rain shower head - how should I disconnect this plug?
There should be a brass or copper block above your ceiling that the piece you're trying to remove is threaded in to. Your picture is showing what appears to be an extended plug. That's not uncommon to see after a plumber roughs in a home. The entire "plug", brass and PVC, should come out and then you'll wrap the threads of your rain head with teflon tape and thread it into the brass/copper block in the ceiling.