Lose oil pressure, how do you change oil pump?

Get a copy of the manufacturer's service manual. It will list all the steps along with drawings and torque specs. You'll need it again when the new pump doesn't solve the problem and you need to change the bearings and probably the crankshaft. It will list the steps for every engine size. We don't know which one you have. Before you do any of that, I'd have the actual oil pressure checked with a mechanical gauge. If it is okay, replace the sending unit. That's a more likely suspect than bearings or the pump.