The newest generation of Pokémon games introduce Terastallizing, which allows you to change your Pokémon's type temporarily, granting your Pokémon a chance to land powerful blows it traditionally wouldn't be able to. But unfortunately, most of the Pokémon you'll catch in the wild will have a Tera type that matches its regular typing — which isn't incredibly helpful when attempting to broaden your team's type coverage. So how do you change your Pokémon's Tera type in Scarlet and Violet?
How to change your Pokémon's Tera type.
Before you can change your Pokémon's Tera type, you'll need to gather 50 Tera Shards of the specific type you want to change it to. Once you've collected enough of them, you should revisit the Treasure Eatery in Medali (where you battled Larry for the first time) and speak to the woman behind the food counter.
So long as you have enough shards of that specific type, she'll let you change your Pokémon's Tera type. It's really as easy as that.
How to get Tera Shards.
Since you need 50 of a certain type of Tera Shard to change your Pokémon's Tera type, you'll have to participate in Tera raids across the Paldea region, or hope to find them in Area Zero. Completing Tera raids is a great way to ensure that you get the exact Tera Shards you need to make the change, as you can pick which raids you participate in.
There are also various NPCs who will give you specific Tera Shards. When you return to Medali and visit the woman behind the counter at the Treasure Eatery, she will reward you with 50 Normal Tera Shards, already giving you enough to change one of your Pokémon's Tera types to Normal.
Some of the professors will also give you Tera Shards if you form a close enough relationship with them. In between the classes you take at the school, you can seek out the various teachers in different rooms. Talk with them and complete whatever requests they have and some will give you impressive rewards.
If you help Ms. Tyme resolve an issue she has with a student who has been following her around the school, she'll reward you with 50 Rock-type Tera Shards, as she used to be a Rock-type Gym Leader. Mr. Hassel also seeks reassurance in his teaching abilities when he's asked to come back home and be his family's lead Dragon Tamer. If you help him, he'll give you 50 Dragon-type Tera Shards.