Some of equipment items can be upgraded to new tier. These items have special label "Can be upgraded to ..." in description.
The upgrading increases some characteristics of upgraded item. You can see difference between item tiers in our library.
For upgrading these items required special Upgrade Rune.
Put item and upgrade rune to Professional Blacksmith window (Reroll window) and press button.
There are big and small upgrade runes which have 100% and 30% chance of upgrading.