Something to keep in mind is you don't get to determine which version of the Android operating system for your phone, that's something that the phone manufacturer (in your case Samsung) and your chosen carrier (Verizon) will determine for you, and when it's convenient for them to do so. And that's a big variable as some phones get more upgraded promptly, some after a few months, and some will never be upgraded. A lot of people mistakenly equate upgrading smartphones with PCs where the user has more options and control over system upgrades and updates but they are two very different platforms.
Which version of Android is your S8 + currently running and what version do you want to upgrade to?
If you've been using your phone to go online, apps works and you can browse web sites, you're already using mobile data (your Verizon cellular account service). So upgrades will be applied whenever Verizon releases S8 + upgrades to you. If you're already using the most current version that Verizon is offering, don't bother trying to force your phone to upgrade, it just doesn't work like that. You have the option to root your phone and then apply a corresponding third-party firmware for your S8 + that may be more current, but that's not a trivial task so be sure to research thoroughly and double-check every step you do. (...and be absolutely sure you have all your data backed up, and don't just assume your backup solution is working. Check that your data is accessible and restorable).
On the chance you're mixing upgrade and update terminology, Verizon does publish an update schedule for their phones. Apparently your phone should have at least the August update patch:
https://www.verizon.com/support/samsung-galaxy-s8-plus-update/?adobe_mc=MCMID=24217370470875540720732628256043622175|MCORGID=843F02BE53271A1A0A490D4C%40AdobeOrg|TS=1602298028&mboxSession=405a71d9d92247d080500f71d1616041