In the latest installment to the Pokémon franchise, Pokémon Scarlet & Violet, you can explore a new ability called Terastallization. This is when the Pokémon becomes a crystallized version of itself with more powerful moves based on its Tera Type. Normally, a Pokémon's Tera Type is based on whatever type that Pokémon is. If you want to change this, you will need to collect 50 Tera Shards and make your way to the city of Medali.
It is important to note that this city's gym leader, Larry, must first be defeated. You cannot change your Pokémon's Tera Type until this has been completed.
How to change a Pokémon's Tera Type in Pokémon Scarlet & Violet
When you reach Medali, and have defeated Larry, you can reenter the gym's restaurant. This location is called Treasure Eatery. Here, you must speak with the chef who is asking if you'd like to change any Pokémon's Tera Type. Whenever you have the required 50 Tera Shards, you can come here and change them out for a dish that can change one Pokémon's Tera Type. This cannot be reversed, but you can switch out the Tera Type again when you have the required amount of shards. Changing out the Tera Type is not limited to any one Pokémon, so you can experiment with different types among your different Pokémon.
For more Pokémon Scarlet & Violet, check out both How to get Bronzor Fragments in Pokémon Scarlet and Violet and How to get Sinistea Chips in Pokémon Scarlet & Violet only on Pro Game Guides.