You should see on your items there’s something a word like Adventurer or Explorer, as well as a number. This indicates which ilvl track it’s on and how far along it is.
In the capital city there’s two dracthyr to the right of the auction house. One of these dracthyr allows you to upgrade items with a mix of flightstones and crests. Flightstones are required for all upgrades, crests are required for the first time you upgrade an item on that slot to that ilvl, the lower the ilvl the lower the type of crest. The first few stages of upgrades (Explorer and Adventurer) don’t require crests, though you should have some whelpling ones from activities around the world you might find a use for later.
There are a few other upgrade NPCs in Valdrakkan but the one by the AH is the easiest to find.