It's easier to do with the manifold off, but it's not absouletly necessary to remove it. You will, however, need an O2 sensor socket which has a slot for the wires to slip thru. Disconnect the O2 harness at the plug, then remove it with the socket. If it's been there for a while, it may take some muscle to get it off, which is why it's easier to do off the car, but it should help if you soak it with some WD-40 or liquid wrench, etc. If you're getting a new stock one, just reverse the process. If not, then splice your new universal O2 onto the plug from the old one and put it on.