This sample repo shows the minimal number of steps necessary to update a RN proj to the new 0.56 version.
Check the commit history for details of the things that I needed to do to ensure that the app was working fine in both Debug/Release builds for both Android/iOS.
Disclaimer: if you use Flow or Jest, the steps in this small TEST repo won't be enough. I suggest you check the 0.56 branch repo for the latest .flowconfig. Even better, I suggest you use the compare version from rn-diff to ensure you update all the correct files.