To upgrade Haki in Blox Fruits, start by equipping Aura. Purchase it from the Ability Teacher in the First Sea for $25,000. Activate Aura to begin earning experience points (exp).
Next, gain exp by taking damage while Aura is active and by dealing damage to enemies. Each hit you deal and take gives you +1 exp.
Equip low-level weapons like dual katanas to minimize damage to NPCs. This allows them to stay alive longer and hit you more times, helping you earn more exp.
Gather multiple NPCs in one place, such as Frozen Village in the First Sea. Let them hit you while you hit them back for efficient exp farming.
Periodically check your progress by visiting the Aura Editor in Middle Town. Click on different stages to see if you have unlocked the next level. Turn your Aura off and on to see visual changes.
Continue farming exp until you reach the required amount (50,000 or 75,000) to unlock full body Haki. With these steps, you can efficiently upgrade your Haki in Blox Fruits.