1. first, you need to install UABE (Unity Assets Bundle Extractor) Here is the link:
(Only download this version, the newer versions are quite harder to use)
2. once installed, you have to open the program and select the "Open" option
3. go to the "gg&c2_data" or folder and lookup for the file Called "Resourse.assets"
4. once the load is complete, you can see a bunch of files, which are:
* Texture2d = the sprites/models of the Characters
* Animati animations
* AudioClip = The sounds of the game
* Sprite = some other sprites (taken from the Texture2D)
* Animator = i guess is the animations of the characters (i'm not sure at all)
* M files that make run the game correctly
* shader = environment settings
* mesh = 3d models
5. extract them, but for that, open "plugins" and click on "Export as PNG"
6. once you extracted them you can edit them
(for that, you need between Photoshop, Paint.net, and Gimp, you can choose every program that you like to modify the sprites, I'll recommend you Paint.net because it's much easier to use)
7. for installing a mod, just open the same file (don't forget to make a backup of it) and look up the file that you want to replace (you can find it by its Id number, Eg: Daisy/Zoey = 160, or Bouncer/Charger = 140) open "plugins" then press "edit" and press "ok", click on the "load" button and lookup for the image that you want to replace, when you did it correctly, press ok when it's loaded.
8. once you finished, press "file" and press "save" call it resourse.assets(any number), and click on "save"
9. once you finished, replace the Main "resource.assets" and replace it with the modded one (don't replace the backup)
if you think UABE it's way much harder to use, I'll recommend you to use "Assets Studio" which can be downloaded here: Github.com (it's easy to use)
Note: this also works on the first game, so don't worry if you ask if it's not gonna work.
Ok, but how do I change sprite?