title: How to rollback just one step using rake db:migrate
body: <p>After adding migration files in the <code>db/migrate</code> folder and running <code>rake db:migrate</code>, I want get back to the previous step, I think using <code>VERSION=n</code> is the right way to do that, but I don't know the correct value of n to use. Is there any command to check the current n value?</p>
  
  <p>It would be great if anyone could provide full instructions on how to use <code>rake db:migrate</code>.</p>