Run it. After that, you can easily upgrade to the new version and merge if you change something. I assume that you are using github, so also click the “watch” button so that you always know about the problems, updates, corrections of this project in the control panel. You can also contribute to this amazing project.
Do not contact directly, because one day they will change something, and this can break your application.
I also suggest looking at LESS, rather than compiling the CSS version of the library, in my opinion, it's easier to configure everything there.
Update:
easily update was probably too strong an expression and was only mentioned about bug fixes and minor updates. In the case of major updates to the version with breaking backwards compatibility there can be no easy and perfect solution. In this case, only if you really think you need this update, can you create a new branch, update the boot twitter there, and then start the painful process of fixing the code. With branches, you can work with the main application with the old and working bootstrap, while part of your team will use refactoring code to become compatible with the new version of the boot file (even if you are only one person in the project, this is also a good idea for testing new solutions in the test branches, so as not to spoil the working code).