I think you can upgrade it from e-bay.
if they provide this facility else the other way is contacting their stores.
Contact any Verizon store and inform them you are out of your contract and want to start a new one with an upgraded phone.
If you are willing to sign on for a longer term wireless contract,
they will upgrade your cell phone no problem.
Just be ready to do so otherwise you may face some problem.
The longer you are ready to sign a contract the easy for your work at Verizon stores otherwise they create lot of nuisance and will take a long time to upgrade your phone.