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.