How to write technical documentation for an iOS application?

Question

Documentation

Hello everybody. I have a slightly difficult situation.
I have in my hands the source code for a client-server application for iOS. Wanted to sell them, but I don't have those. documentation for them.
The application is written in objective-C.
The question is: How to make it?
The documentation needs to describe how to change the design (well, it's not difficult, replace the graphics and audio) and change any functions. Unfortunately, I do not have such programming skills to understand how everything works. Also, there are no funds to hire developers.
prntscr.com/781sw5
prntscr.com/781t3q client
prntscr.com/781t7d server