Use an anvil and combine the damaged fishing rod with another fishing rod
Use an anvil and combine the damaged fishing rod with a matching enchanted fishing rod
Use a grindstone only if you want to remove enchantments first, then repair by combining rods on an anvil
Use the Mending enchantment and gain XP while holding or wearing the rod
Use the Unbreaking enchantment to slow durability loss
Craft a new fishing rod if the old one is too damaged and you do not want to spend levels
Use the crafting grid with two damaged fishing rods to make one repaired rod, if applicable in your Minecraft version
Repair the rod at a smithing table only if your version or modpack specifically supports it