Place the required materials in your crafting grid:
6 Iron Ingots
1 Netherite Ingot
Open the crafting table
Arrange items:
Put 5 Iron Ingots in a U-shape (top row: Iron, Iron, Iron; middle row: Iron, empty, Iron; bottom row: empty, empty, empty)
Put 1 Netherite Ingot in the center slot
Take the completed Netherite Armor piece from the output slot
Equip the armor piece:
Drag it into the armor slot in your inventory
For different pieces:
Helmet: 5 Iron Ingots + 1 Netherite Ingot
Chestplate: 8 Iron Ingots + 1 Netherite Ingot
Leggings: 7 Iron Ingots + 1 Netherite Ingot
Boots: 4 Iron Ingots + 1 Netherite Ingot
To upgrade existing armor instead:
Place the armor piece in the smithing table left slot
Place the Netherite Ingot in the right slot
Take the upgraded Netherite armor from the output slot