You can Ascend your Primals to raise its max Level in the same menu where you Level up your Primals. The first Ascension costs Coin, which you can earn in the Gilded Castle Arena. Subsequent Ascensions require Ascension Materials which you can get from the Hall of Mirrors Arenas. There is a different Arena for each Essence, so you must do the Arena that matches your Primal’s Essence. Currently, the max Level cap is 80.