The process is called "disassembly". You can do it by hand using reference tables if you really want to, although this is only really practical for small pieces of code. The process can be automated to some extent using a program called a "disassembler", that supports the particular processor architecture the code was written for (in this case the Zilog Z80). They aren't fool proof, and will usually need some manual intervention in order to produce readable code eg. giving meaningful names to labels, identifying what is code and what is data. The more sophisticated disassemblers actually execute the code on an emulation of the target machine in order to determine whether particular sections of memory are code, data (or even both); they can even identify specific memory areas and give hints as to what the actual function of a routine is. eg. if it's writing to video RAM, reading input ports, writing to sound registers etc. Personally, I use SkoolKit for Z80 disassembly. It's primarily designed around disassembling ZX Spectrum programs, but it's open-source and can be adapted to most Z80 based platforms with a bit of work. However, for a first attempt at disassembling the Pac-Man arcade boards, you may want to start with a more generic Z80 disassembler.

@@gwishart Wow, thanks so much!

Clyde was one of the ghosts in Pac-Man. This video is about Ms. Pac-Man, the ghosts are: Blinky, Pinky, Inky and Sue.

@@gwishart OH, I just thought they always had the same name lmao

Yep, Ms. Pac-Man was the first successful ROM hack :)