It can be tricky to do without damaging the receiver but if you don't care about the receiver just put it in a bench vise (clamping on the trunnion) and use a big wrench with an extension arm to unthread the barrel. A little heat on the trunnion can help but only a little. If you want to save the receiver you will need to make up some sort of trunnion block/receiver protector. I used a cut length of grade 8 bolt and some aluminum to support and protect the receiver at the trunnion when I changed the barrel on my old PE90. Do not put any clamping force on the receiver body, only the trunnion.