If you need to change a Pokémon's Tera type, go into the details section (level/shiny/gender) in the builder. The default Tera type is your pokémon’s primary typing.
The sentence above tells you where to go to change your Pokemon's Tera type. So, to get Flutter Mane's tera to become Normal, you would first click the details section in teambuilder. After that, you will see Terastal Type: followed by a white box that says (automatic type). Click on the white box to show all of the types and click on Normal. That will allow your Flutter Mane's Tera type to become Normal.
Sources: /rfaq tera in the Monotype Room on Showdon(for the quote) and experience with changing a Pokemon's Tera type on Showdown.
Hope this helps.
