You want to start over. Let’s say you choose the wrong starter Pokémon for example. You already looked in the settings menu of the Pokémon GO app and you couldn’t find any way to reset your progress back to the start of the game.
It is currently not possible to entirely reset your game progress. But there is a workaround.
Instead of trying to resetting your game status, you can choose to create a new account. For example, if you used your Google account to access the game, you can now opt to use a Pokémon account. However, in this case you shall also have to invent a new username – which may be hard since many names are already taken by other players.