Use a smithing table with a netherite ingot and the damaged netherite armor piece
Use an anvil with netherite ingots to combine the damaged armor with another same-type netherite armor piece
Use an anvil with the damaged netherite armor piece and a repaired or undamaged same-type netherite armor piece
Use the Mending enchantment and gain experience to repair the armor automatically
Use a grindstone only to remove enchantments, not to repair the armor