That depends on your carrier. Some are pretty good, technically, and can reflash the update for you. Some are just stores that sell phones and accounts, and the associates know less about phones than you do.
If you can find your variant at
XDA, see if someone has the current update file for your variant posted (or someone can post it for you) - then you can use Odin to flash it.
I assume that the phone was using the stock unrooted ROM. (If it was rooted or using a custom ROM, you'll have to reflash the stock ROM, and if you don't have everything backed up you've lost it.)
