Best Practices for Extending Rails Applications - Help

Using Sharetibe Go which is a Rails app

What is the best way to modify Sharetribe Go source code for our charity marketplace that would still allow for easy integration and merging of future releases from Sharetribe?

I’m a C# .Net programmer that is new to Rails. We want to deploy Sharetribe Go on a hosted server and make a series of functional modifications to it. Being new to rails I am trying to find out the best practice for making fairly moderate functional modifications and still being able to keep somewhat in sync with future releases of the Sharetribe Go base code.

Any guidance or links to learning resources on how to best do this would be appreciated.

Thanks so much for your advice. - Susan