My suggestion: buy a special socket for your socket wrench. Get a special metric Allen wrench socket, (I think it is a 6 millimeter you need). You may need a regular 6 mm Allen wrench also, as the clearance may be too tight for some of the bolts. Unscrew the Allen bolts. Unscrew the big bolt in the center of the diaphragm (13 mm I think). Install new diaphragm using the bolt--there is a front and a back--the bolt will not go through from the wrong side. I think the label on the diaphragm goes to the inside. Rotate the pump using the starter cord, (being careful to not start motor), to help seat the diaphragm into the groove. Hammer on it gently with a wooden mallet if needed. Reinstall the pump head. Same on other side. Try placing a pan underneath to catch the oil as it runs out. Replace with new oil (30 w). Be sure to close the tank valves first. Wear gloves due to pesticides.
10 minute job--takes me about an hour. This is a good time to check for damaged valves or broken springs. Tighten the bolts in a criss cross pattern.