Get Diamond armor pieces
Find Ancient Debris in the Nether
Smelt Ancient Debris into Netherite Scrap
Craft Netherite Ingots from Netherite Scrap and Gold Ingots
Use a Smithing Table
Place one Diamond armor piece and one Netherite Ingot in the Smithing Table
Take the upgraded Netherite armor piece
Repeat for all armor pieces you want to upgrade