Doing a mapper hack requires you understand what mappers actually do. And for that you need to be reasonably comfortable with assembly.
For Converting UNROM to MMC1... the short version is this:
1) Cut into the game's Reset routine and prep the MMC1 to be in the proper mode and whatnot (general rule is write to all mapper regs at least 1 to get them in a known state)
2) Find all instances of the game doing a write to an UNROM reg and replace it with the equivalent MMC1 version of that write.
That's pretty much all there is to it. If you understand assembly and are familiar with NES architecture, it's not particularly complicated. But if you're a newbie to all this stuff, it'll probably be super confusing (but still possible).
NROM is simpler because it's effectively "no mapper" which means you can skip step 2. In fact it's pretty easy to create a generic "shell" to convert any NROM game to an MMC1 game.
Tools you'll need:
- Documentation on UNROM
- An injection assembler, like xkas, bass, (or schasm
You can ignore the assembler and do everything in a hex editor if you want. But then you have to manually insert opcodes... so you'll need a 6502 reference sheet
Again, even though a veteran could probably crank this out in less than an hour, for a novice this is going to be quite a difficult task because there's a lot you have to learn and get familiar with.
Start small. NROM is much, much easier, so I'd start with that. Convert something like Donkey Kong or something.