When you’ve collected enough Mega Energy, go to an eligible Pokémon’s summary page and tap “Mega Evolve.” You can also Mega Evolve a Pokémon from the Raid Battle Lobby or when selecting your GO Battle League Battle Party.
Mega Evolving a Pokémon for the first time costs a significant amount of Mega Energy, however, if you Mega Evolve a specific Pokémon multiple times, it will require less Mega Energy over time. Different Pokémon species may require different amounts of Mega Energy to Mega Evolve.
After being Mega-Evolved, a Mega-Evolved Pokémon will stay in its Mega-Evolved form for 8 hours, after which it will return to its regular form.
Note: Certain Pokémon like Charizard have multiple Mega forms. When Mega Evolving a Charizard, you can choose to evolve it into Mega Charizard X (a Fire and Dragon-type) or Mega Charizard Y (a Fire and Flying-type).