React Native code can be edited using any known text or code editor. Here is a list of recommended code editors:
Webstorm: This is commercial and one of the best IDEs for Jave Script . To buy click here
Atom Editor: A free and open source tool with awesome features. Download here
Deco IDE: Another free and open source IDE that supports React Native. Download here
Nuclide: Is built as a single package on top of Atom to provide hack ability and the support of an active community. Download here
You are required to download an IDE to open project's source code and start editing it. Many free IDEs are available for free and paid IDEs would definitely have extra features.
We felt very comfortable with Atom Editor and it was used for building this project.