How do you change the oil pump in a 1988 Mitsubishi Starion Is the oil pump internal or external Can a do it yourselfer fix it or does it need to go into the shop?

Oil pumps are internal, they are inside the oil pan, at the bottom most point of the engine. But before you replace the oil pump, recognize that oil pumps are designed to produce a stream of pressurized oil into a set of oil lines that distribute pressurized oil throughout the engine to bearings and sometimes hydraulic lifters. A pressure relief valve in the oil line prevents the oil pressure from going too high by releasing oil if the pressure starts to go above the pressure setpoint. But that is not typically the problem. Most low oil pressure conditions are caused by worn bearings that result in too much of a gap through which oil escapes the pressurized lines. If the gap between the bearings and the crankshaft is too large, oil will rapidly leak out, dropping oil pressure. While it's always a good idea to replace the oil pump when you rebuild an engine, it usually isn't necessary to replace an oil pump at any other time. In fact, if the oil pump fails, the engine is usually trashed almost immediately, then you MUST replace the oil pump. May I suggest that you change to a heavier grade oil before you attempt to solve a low oil pressure problem by replacing the pump. If you've been using 5W15 oil in the engine, change to 10W30 or even 15W50 if you can find it. Just try an oil with higher numbers to see if the oil pressure problem will be somewhat improved. The auto manufacturer has suggested an oil for that engine but understand that the suggestion is based on a NEW engine, not one that is worn. Once the engine ages you need to find ways to keep it running that don't involve a lot of expense.