Instructions
Tap "Pokemon" on the bottom screen.
Place Pupitar into the first slot on your Pokemon lineup. Tap "Pupitar," select "Switch" and tap the first Pokemon in your lineup to make Pupitar the first Pokemon.
Battle other trainers and wild Pokemon to gain experience. You'll find the amount of experience Pupitar has, as well as the amount he needs in order to gain a level, listed under "Summary" from the "Pokemon" menu. The higher the level of your Pupitar, the more experience he requires to level.
Earn enough experience to reach level 55. At level 55, Purpitar will evolve into Tyranitar automatically at the end of the battle.