Upgrade ruby from ruby-2.0.0-p451 to 2.2.2 on Mac

In our system we are using RoR, when ruby version is ruby-2.0.0-p451 and rails > 3 Our plan is to upgrade rails to 4. When googling for information, we found that it will be smarter to start upgrading ruby first to avoid blocking gem upgrade (so that we will have full support for the rails versions. If we don’t upgrade ruby version, it might block the upgrade of some of the gems needed for rails 4.0 and above.)

So we have decided upgrading Ruby to 2.2.2 first.

Now, I'm trying to understand how can I upgrade to 2.2.2 version by looking for the differences, and map the changes we have to do, but can't find this info. Can someone assist me by pointing me to right location where can I see the features and changes that have to be done in our code? Thanks!