MilCop, go to your UserCP on the blue banner near the top of the page. To update your vehicle in your signature, go to edit signature, make the change, and then hit "Save Signature".
If you want to change the vehicle in your profile, got to "Edit Your Details", and go down near the bottom of the page to see your vehicles. Hit "Save Changes" and that should do it.
Chap , DW Joy, and Fur Baby Sango
2006 Ford F150 Super Cab 4x4
2008 Surveyor 263