How to undo an incorrect rails migration after rake db:migrate

class User < ActiveRecord::Migration
  def self.up
    create_table :users do |t|
      t.column "username", :string
      t.column "fname", :string
      t.column "lname", :string
      t.column "email", :string
      t.column "hashed_password", :string
      t.column "campus_id", :integer
    end
  end
 
  def self.down
  end
end