If you want to level jobs up in Bravely Default 2, you will need to earn what are called Job points; here's how and what you need to know.
How to Farm JP in Bravely Default 2
To farm JP and built it up in Bravely Default 2, you will need to fight battles and win them; every time you win a fight, you will earn a small amount to leveling up a job. If you want to level a specific job, you will also need to have it selected and assigned as your primary, as jobs will not level up or earn JP well in the secondary slot.
That said, if you want to farm JP, we recommend doing so in dungeons, as you will find a high concentration of enemies in them. Along with this, try to go for enemies around your level, as these will offer up the most JP, including XP.
As a side note, you can farm real easy early on when you get to the point where a powerful NPC called Sir Sloan joins your party. We recommend going around and attacking every enemy in sight while he is in your party.
This is because Sir Sloan will both one-shot formidable enemies and heal your characters if they pass out, making it a cakewalk to farm both XP and JP.
But beyond this point, you should be careful as certain enemies may prove challenging; because of this, we advise staying as close to your current level as possible to avoid unnecessary death or failure.
For more on Bravely Default 2, we at PGG have you covered with awesome and informative guides like how to change jobs and how to change voices to Japanese in Bravely Default 2.