Sure, no problem.
After you reach level 78, you have to talk with the rebirth manager.
After a short dialogue, you'll have an option to make a rebirth or not.
After accepting it, your level will become 1 again, and you will get 2 items.
One of em is required to learn a new passive skill at the skill-learning NPC, while the other will be used as a passcard to enter the upcoming tartarus tower.