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
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 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)
ruby bin/rake db:migrate
heroku run rake db:migrate
More information can be found on the Active Record Migrations section of the RailsGuides site.