To change your Rails models, you should create a migration containing the change and execute the migration.
First, generate the migration. From within your app directory:
rails g migration DESCRIPTION
Edit the file, which has been created in db/migrate
Define your desired changes. You can place them in a function change (with reversible if you want to be able to undo the change), or place them in functions up and down, where up defines what should be changed and down defines how to return to the previous state if you decide to roll back this migration.
If you want to make the change as a SQL query, you can use:
execute <<-SQL
YOUR SQL QUERY
SQL
Then on the command line run:
(OS X Terminal) rake db:migrate
(Windows CMD) ruby bin/rake db:migrate
(Heroku CLI) heroku run rake db:migrate
More information can be found on the Active Record Migrations section of the RailsGuides site.