Hi there Virginia and welcome
you will need to get an upgrade disk to do this and can get one from my link below. I have recommended windows 7 though as it is much better then Vista was, and around the same price
One thing you need to do before you make the purchase though is run a (free) tool to ensure you computer is capable of running windows 7 (they it has at least the minimum specifications).
You can download and run this tool from my link below
If you would like any additional information or assistance, please do not hesitate to let me know.
Else if you can take a second to rate my service by clicking one of the stars at the top of the screen then submit, that would be greatly appreciated.