To level up and improve your skills, you will have to accumulate XP, and to level up the jobs you will have to accumulate JP or job points, which you can obtain during battles and by completing missions.
Keep in mind that by winning a battle, you receive some XP and JP as a reward, and the more JP you have, the faster you will level up the jobs.
Leveling up a job will allow you to access a new skill that you must level up to a maximum rank of 12 to obtain all the skills you can from a job.
The best way to level up a job is by farming on the world map and drawing the attention of enemies on the map to finish them off. Although you will also find a good number of enemies in the dungeons.
You can also get XP points from experience orbs, as well as from chests, expeditions, and in side quests. That's everything you need to know about how to level up jobs in Bravely Default 2, so now that you know all of this, we hope you can level up in all your jobs as much as you need to.