My only experience is with Uberti '51 Navies and a Euroarms Rogers & Spencer. The Uberti's were a press fit. Grab it with the pliers & it should pop right out. Twisiting while pulling would probably help. The Euroarms Rogers & Specser was threaded in. Grabbing with pliers & twising couterclockwise removed it. Bought a replacement threaded sight for that. I have no experience with Piettas, so can't say for sure; but think they're press fitted like the Uberti's.
FM