You don't need a data plan to update it. All you have to do is download blackberry desktop software on your computer.You do need to have internet on your computer.
1-Connect your blackberry to your computer using a USB cable
2-Click "Device",
3-Click "Update"
3-Follow instructions
The software will search for updates. If there are not any updates available at the moment, check again in a week or 2.
You could also update by asking a friend of yours to let you put his or her simcard in your phone and do a wireless update.
Hope this helps!