In Diablo II, upgrading unique and set items allows you to improve their base item type, which increases their base stats, such as defense (for armor) or damage (for weapons), while retaining their unique properties. This can be especially useful for making powerful low-level unique items more viable in the late game.

To upgrade unique items, you need to use a specific Horadric Cube recipe. There are different recipes for upgrading unique items based on whether they are Normal, Exceptional, or Elite quality.

Recipes for Upgrading Unique Items:

1. Upgrading a Normal Unique to Exceptional

• Weapon:
  • Recipe: 1 Ral Rune + 1 Sol Rune + 1 Perfect Emerald + Unique Weapon
  • Result: Upgrades a normal unique weapon to an exceptional unique weapon.
• Armor:
  • Recipe: 1 Tal Rune + 1 Shael Rune + 1 Perfect Diamond + Unique Armor
  • Result: Upgrades a normal unique armor to an exceptional unique armor.

2. Upgrading an Exceptional Unique to Elite

• Weapon:
  • Recipe: 1 Lum Rune + 1 Pul Rune + 1 Perfect Emerald + Exceptional Unique Weapon
  • Result: Upgrades an exceptional unique weapon to an elite unique weapon.
• Armor:
  • Recipe: 1 Ko Rune + 1 Lem Rune + 1 Perfect Diamond + Exceptional Unique Armor
  • Result: Upgrades an exceptional unique armor to an elite unique armor.

Important Notes:

• Item Level: When upgrading an item, its item level does not change. This means the item's level requirements and properties will remain the same, but the base item type (and thus the stats like defense and damage) will improve.
• Rune Availability: Some of the runes required (such as Pul, Lem, or Ko) are mid-level runes, which may take time to acquire, especially for upgrading items to elite status.
• Stat Scaling: When upgrading an item, the base stats (like damage for weapons or defense for armor) increase according to the new base item type. The other unique properties of the item (such as increased attack speed, life steal, etc.) remain the same.

Benefits of Upgrading:

• Higher Damage/Defense: The primary reason to upgrade is to increase the base damage or defense of your items, making them more effective in higher difficulties (Nightmare or Hell).
• Maintaining Unique Properties: You can keep the powerful abilities of a low-level unique item (such as life leech, resistances, or enhanced damage) while giving it stats comparable to higher-level gear.

Restrictions:

• Ladder-only: In Diablo II: Lord of Destruction, upgrading unique items was originally restricted to Ladder play (online mode), but in Diablo II: Resurrected, these recipes are available in all game modes.
• Set Items: The upgrade recipes only work for unique items, not for set items.

Here I upgraded two melee weapons for example and they become with better damage and higher requirements.