It's pretty easy. Three nuts and a paper gasket keep the pump flush with the Injector pump. First remove the fuel filter line. Next remove the primer pump, you should then be able to remove the plastic line compression fitting to the secondary filter with a flare nut wrench. Loosen those three nuts and back the pump off the studs.

Once you remove the pump, you want to remove the flare fit/mount, the spring and valve is underneath. On the bottom of the lift pump under the port you connect to the primary fuel filter, is another nut that gives you access to another spring and valve. On the back of the pump, there's a roller/cam lobe/tappet (forgive me) that interfaces with the IP that you can disassemble and remove. Once you remove the plunger, deep in the bore is a tiny o-ring you can replace (keeps oil and fuel from mixing). The giant hex head in the front of the pump contains the main pressure spring. Apparently, if your spring is not broken (99% likely to be fine) it's a good idea not to mess with removing that cap to replace the crush washer -unless of course it's leaking which is equally unlikely. Getting all this stuff off the pump when it's not mounted is a b_stard. Lots of soft wood in a vice so you can crank down on the nuts with a breaker bar and deep sockets. Just be careful when you're mounting the pump in the vice that you don't clamp down on that flange (the one that mounts to the IP) as apparently it's a little fragile.

It's an easy job, just a little annoying because some of those valves/nuts/attachments are on pretty tight. I had a nearly impossible time removing one of them -nothing fit but a crescent wrench which stripped painfully with every turn. Later had help with an angle grinder removing a small bit of metal to fit a socket, then no problem.
When you reinstall the pump, use Gasgacinch or permatex high tack sealant on the studs and the gasket -mine leaked with the new gasket alone.

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1981 240D 4sp manual. Ivory White.