Update file field..

Hello champs,

I have uploaded a file and saved it in tmp/ location. But now i need to
update this file field. I mean i need to replace that file with another
file.
While uploading, i used this:-
@Resume = Resume.new(params[:resume]) .. This will call my model
function upload_resume=() which will actually upload file.

Can you guys tell me/light me/tutorials that how should i update it?

Thanks in advance to you people.