That's not really specific. Is that an old plan? Also how do you add data to a plan with unlimited data? Can you give the specific name of your plan?
While "$50 unlimited" is not the name of any of Verizon's plans, it appears you are looking for a "data boost" for after you hit the limit before having the possibility of being de-prioritized. You can add a data boost from the Verizon app on your phone.
For future reference, the differing Verizon Unlimited plans would currently include the following:
Verizon Plan Unlimited
The cost of these plans is dependent on how many lines are on your account so simply saying "$50 unlimited" doesn't mean a whole lot. For that price, I believe you can get Go Unlimited, Beyond Unlimited and maybe even Verizon Plan Unlimited depending on how many lines you have on your account.