1. Save the attachment to your computer. Right click on the attachment, select "Save As..." and save it to a place you can easily find it, like your desktop or My Documents.
2. Go to the location you saved it to, open it, make any changes you need to make (you do have Microsoft Word, right?) then save the file.
3. Open the original email, click on Reply, enter what ever info you need to in the body of the email, then click the "Attach File" button/menu item (it depends on what program/version email program you are using as to where exactly this menu item is. On some it's a paper clip icon, others it just says "Attach File") Browse to where you saved the Word file, and select it. You should now see the file attached to your email and you can send it on.
I'm sorry I can't give you specific details because I don't know what software you are using, but this should give you a general idea. Basically, save the attached file to your computer, modify it, then reattach it to a new message and send it back.
If you don't have MS Word, go to www.openoffice.org, and download and install OpenOffice. It's free and can edit/save MS Word documents.
Good luck.