Subscription plans cannot be changed directly. You will need to order a new annual plan(s) and then cancel your old monthly plan(s). Your data is not associated with a plan, so changing a service plan will not affect your files. (Just make sure you have enough storage space and download bytes).
Some users may have multiple monthly service plans; in this case, you need to do so for each monthly plan.
If you ordered an annual plan, please don't cancel it and order it again. You just need to make a payment of the annual amount. To find your annual service price, please log on to www.drivehq.com, go to My Account page, then click Transaction History -> View Order List. If you have multiple (active) orders, you need to add the total amount.