If someone sends you an email with a Word or Excel file attachment you're supposed to revise, first save the attachment to your computer before you begin editing it. Then — when you're finished editing it — attach the revised document to your email reply.
If you instead work directly on an attached e-mail file, chances are good the edits you made will be ignored, and when you reply all the person will see is the file as he or she first sent it to you.
Also, you might consider renaming the file so the person you're sharing it with can keep the two documents separate on his or her computer. As an example... as part of a 6-person panel presentation at the 2010 WLA Conference, I was sent a presenter's agreement form which I needed to fill in & submit to the MATS chair. The file name of the original un-edited document was "2010PresenterAgreementForm", so I downloaded the original form and filled it in, and I saved it as withe the new file name "2010PresenterAgreementForm-JoySchwarz". That way the MATS chair can keep this version of the form separate from the other 5 versions of the form he'll receive from the other presenters on the panel!
Need a refresher on how to open & save email attachments? Read this previous Ides Email Attachments: How to Preview, Remove, and Save.