Customizing open source code

I have to customize an open source client-server application for editing documents. The code is under license that allows code modifications, but the organization behind the project makes source and binaries available with limitations on simultaneous connections. I believe it's a variable on the code (or something like that).

I need documentation for operating procedures to a)change the code and b)deploy the solution, including the terminal commands to use.

Please, consider the following information:
-Main application language: Javascript
-Operating system for application server deployment: Debian 11
Further information will be passed on later.