Canvas-lms update rails 2.3.5 to 3.2.12

I am going to work on the open source Canvas-lms. Its using rails 2.3.5 but i am familar with rails 3.2.12.

1. How to upgrade rails 2.3.5 to rails 3.2.12.
2. What are the challenges.
3. What is the best way to do it.

Include this in your Gemfile:

gem 'rails', '3.2.12'

Then run the command

bundle update rails

I think this is the best way to do it.