start by draining the oil. Pull the head and plungers off. drive a long thin screw into the seal and pull it out using vise grips. just be careful not to drive the screw into the piston or you will score it and the seal will always leak. lube up the new seal and press it in, I usually use an appropriately sizes deep socket to seat the seal.
OR you can pull the pump off of the pump stand, remove the head and plungers, remove the rear cover and pull the crank and connecting rods and push the seal out from the inside. install new seal. look at all the parts that you know you can never get back in properly, give up and hit the local bar