so the answer to this is a two parter:

if your wii's homebrewed and you're launching your PMs from a usb loader (like usb loader), then the process is space and time consuming; there's a PM iso compiler that people used to use for netplay to build a consistent PM iso so netplay would be sync'd up. unfortunately, that method's been phased out of netplay, being replaced with a more modular, changeable SD card loader. The gimmick here is that you need to find a PM 3.6 iso builder with instructions. Then, from there, all you would need to do is replace whatever "projectm" folder exists for that compiler with the one you'd want to build. I honestly wouldn't recommend this option, because not only do you need to make sure the PM compiles correctly, you also need a USB drive with enough space to hold all of your 7+ gigabyte PM isos. This, while having easy access to all your PM builds, makes the collateral for getting each build on your wii more difficult.

The second, and easier overall, method is, simply renaming files on your sd card. There's a few caveats and different pieces here; if you're using the stage builder, non-homebrew method to load PM, you've gotta use the 2gb SD card, which means that unfortunately you're gonna need to delete and replace files every time you want to switch builds. But, if the setup you're using now involves a bigger card, then the process is as simple as renaming some folders. Not just the "projectm" folder, mind you; you download "Jimbo's PM build" zip file, it should come with a bunch of folders ("projectm," "codes," "apps," etc). If you can fit multiple PM builds on the card, it's important you keep these grouped up (jimbopm, jimbocodes, jimboapps), and rename them all when you decide to switch

Basically, to have a bunch of modular PM builds requires a drive with enough space to hold all the Isos, the patience to compile all of them, and the ability to rename them all to distinguish them. If you can homebrew-load brawl, you can just get a decent sized SD card and rename them as needed. That's that