By default, your month will run from the 1st to the 1st (normal calendar months). You don’t need to do anything for it to work that way.
However, if you’ve shifted your month to match your payday already, it’s not currently possible to go back to calendar months. We’ll add this soon.
You can force it back next month, by transferring £200 or more to your account on June 1st, then using that payment to change your start date again.