I’m a beginner. May I ask how to upgrade quickly?

[Dynast-Kid]

I don't know if this is a spoiler or not, but once you reach (SPOILER)Mt. Bur-Omisace (which is about halfway through the game, so you've got a while), you are able to by the accessory Golden Amulet. These double the amount of LP you earn from battle, and are a good way to earn License Points!

There's also an accessory called the Embroidered Tippet which doubles the amount of EXP. you get, so stay on the lookout for that!

Also, you can seriously beat the main game on like, level 40. This is definitely a game where you want to level up between missions. If you level your characters up about 3 to 5 levels before each destination, you shouldn't have any problems (it might even make the game too easy). It all kind of depends if you like to be well prepared for each battle, or if you like to be challenged.