Use a smithing table with a netherite ingot to upgrade diamond armor to netherite armor
Use an anvil to combine two damaged netherite armor pieces of the same type
Use an anvil to combine netherite armor with a diamond armor piece of the same type
Use a grindstone only to remove enchantments, not to repair durability
Use the Mending enchantment and collect experience to repair netherite armor
Use the Unbreaking enchantment to slow durability loss
Use a crafting table with a netherite ingot and a damaged netherite chestplate only in older versions if applicable
Use the repair and rename function in an anvil with netherite ingots or matching armor pieces