If you've been playing in D2R for a long time, then you should know that to upgrade d2r items, you should first make the Horadric Cube recipe. The same goes for upgrading rare or unique items, which requires crafting a specific Horadric Cube recipe.

Unique items Each recipe is also different. But it always contains a perfect gem, except in the Hellforge where you can always get one, it doesn't drop on normal difficulty. They have a chance of randomly dropping from Act 1 of Nightmare for a chance to get a random Perfect Gem.

Upgrading a rare or unique items will elevate the base weapon from Common to Superior, or from Superior to Elite. Therefore, its basic stats, such as damage and defense, will be improved. If those base stats Diablo 2 Resurrected Runes are affected by percentage based modifiers, then upgrading does make a big difference. 

RPGStash will give you the following recipe : 

Rare Items (yellow)

Normal rare armor to Exceptional

    Normal rare armor
    Thul rune
    Ral rune
    Perfect Amethyst

Normal rare weapon to Exceptional

    Normal rare weapon
    Amn rune
    Ort rune
    Perfect Sapphire

Exceptional rare armor to Elite

    Exceptional rare armor
    Pul rune
    Ko rune
    Perfect Amethyst

Exceptional rare weapon to Elite

    Exceptional rare weapon
    Um rune
    Fal rune
    Perfect Sapphire

Unique Items (gold, which looks more like pale brown)

Normal unique armor to Exceptional

    Normal unique armor
    Shael rune
    Tal rune
    Perfect Diamond

Normal unique weapon to Exceptional

    Normal unique weapon
    Sol rune
    Ral rune
    Perfect Emerald

Exceptional unique armor to Elite

    Exceptional unique armor
    Lem rune
    Ko rune
    Perfect Diamond

Exceptional unique weapon to Elite

    Exceptional unique weapon
    Lum rune
    Pul rune
    Perfect Emerald

If you already know these recipes well and don't have time to collect them, then you can Buy Diablo 2 Resurrected Runes from rpgstash.com, which will be the most efficient way. Of course, you can also buy d2r items directly, and you can also limit the quality attributes of items, which is very convenient for players. Order now and you will get Bonus XP +50% bonus.