An Upgrade Station allows items with available upgrade slots to be fitted with various buffs for a price. Upgrade Stations can typically be found at any town blacksmith.
If you walk over an item with upgrade slots, chat will print: "You notice an upgradeable <item name>." Upgrade slots can be examined by using the "appraise" command on the item in question.
Upgrades are separate from enchantments. For each slot, one upgrade may be purchased (ex: +1 Agility, but not +2 or more). Upgrades up to +10 must be found as loot or purchased from shop or Yojimbo pawn shop machines. It is recommended to pick the most powerful upgrade type given this limitation (+FW, death resistance, and so on). There is no way to remove upgrades at this time.