Choose a type of item:
In Ragnarok Online you can upgrade both weapons and armors. To do so, you have to go to one of the Upgrader NPC with your weapon/armor, the appropiate Upgrade item and some zeny to pay the fee.
To upgrade an armor, you need an elunium. You can safely upgrade an armor up to 4, after that any further upgrades are not guaranteed, all you have is a chance of upgrading to the next level. The chance lowers as the armor upgrade gets higher.
You can upgrade weapons provided you have the right stone. Weapons level 1 need Pharcon and level 2 need Emveretarcon, both are sold by a NPC next to the upgrader. Weapons level 3 and 4 use Oridecon, which is dropped by several monsters.
You need to be able to equip the item you want to upgrade. A wizzard will never be able to upgrade a Full Plate because they can not equip it.
As of Armors, you can only upgrade items equiped in: upper headgear, armor, shield, garment and shoes. Note that accesories and mid&low headgears are NOT upgradeable.