Use an anvil to combine two damaged armor pieces of the same type
Use an anvil to combine damaged armor with the same armor item to repair it
Use an anvil to combine armor with the Mending enchantment and gain XP to repair it
Use a grindstone only to remove enchantments, not to repair armor
Use a crafting table to combine two damaged leather armor pieces in some versions
Use the Smithing Table for upgrading armor in newer versions, not repairing it
Use XP orbs while wearing Mending armor to restore durability
Use the /repair command in creative or with cheats if enabled
Use a villager or loot source to get a replacement armor piece if repair is not possible