Edit Gemfile and add the version you need:

gem 'rails', '~> 6.0.0.beta3'
After in your terminal you can run

bundle update
Finally run

rails app:update
This is a wizard task that help you to upgrade files.

If you use, like us, haml-rails you cant use it.
We have to wait the patch for compatibility (20 Marzo 2019).
You can use directly haml with the core gem haml