How do I regenerate images after resizing them using rmagick

I have a site with previously loaded images at various height and
widths, I changed what I wanted the height and width to be in my model
but the images have not been re-sized according to my specification.

How do I get them to be regenerate to the newly specified size?

My model code:

"class Image < Doc
file_column :file, :magick => {:image_required => true, :max_width =>
“800”, :max_height => “2000”, :versions => {:thumb => {:size =>
“300x300!”}, :mini => {:size => “200x200!”}}}

validates_file_format_of :file, :in => [“gif”, “png”, “jpg”],
:message => “has to be gif, png, or jpg.”