it's an rpgmaker mv game, don't know much about modding them, but it doesn't seem difficult
graphics directory : (game directory)\www\img
audio directory : (game directory)\www\audio
maps, and other data directory: (game directory)\www\data\
audio and graphics aren't in an easily readable format, it takes a 2 second web search to find an rpgmaker mv audio/image decryptor, the data isn't encrypted tho
data is just readable with whatever text processor you use, theres probably a better tool for editing the maps than using a text editor
if you wanna change the entire game, it might be easier to just use rpgmaker mv (it was free for a bit iirc)