It's pretty easy. You'll need a USB cable, though.
1. Find a copy of QPST (for legal reasons, I can't post a link to it here, but it's pretty easy to find on Google).
2. Find a copy of the old software you want to flash to (since you have a V3m, try
here).
3. Connect your phone to your computer.
4. Launch QPST.
5. Select the Software Upgrade option.
6. Select your phone and the file you downloaded in step 2.
7. Enjoy your phone!