First you need to have your PSP setup where you can dump your files off your UMD onto your PC (Some homebrew versions have it built in the menus to dump automatically when selected). Then you open it with a program, such as UMDGen. Then you can read the files in the UMD dump data, select what you want to explore (example assetsfb to change character skins, moves, etc) and extract it. Open the extracted file, usually with Winrar to then extract individual files once again to either edit via notepad or to hex edit, all depends on what you're wanting to do with what files.
Hope that helps.