How can I change texture to the character model in which iso games in psp?

[runesword]

These days I was modifying a texture of the game the 3rd birthday, in the PPSSPP emulator it is super easy to replace that texture, but in the psp itself I have no idea, I know that the iso can be decompressed and then there is a folder called USRDIR where it is saved the textures and models but I have no idea how to decompress those files and replace the texture, an expert helps you here: D