On this page of the FNAF Security Breach guide, you will find a walkthrough for the Party Pass quest.
After you upgrade Freddy, you will have to face one of the animatronics - Chica or Monty. To reach them, however, you need to have a Party Pass, which allows you to enter the new areas.
The pass is in Chicka's room. First, leave Parts and Service by using the elevator behind the door with Roxy drawn on it. It will lead you to her room. Use Freddy and go to the room where it all began.
This time, get into the ventilation shaft. It will lead you to Chica's room. Keep in mind that the room is at the third ventilation grille which you'll encounter on your way. Moreover, watch out for a miniature Music Man patrolling the shaft. ,
As you reach the room, approach the dressing table. The gift box containing the Party Pass is on it.
After you leave Chicka's room, Moondrop will appear. You now need to get to the charging station as fast as you can. The closest one is in Freddy's quarters.