To change the default story format, select its card in the Story Formats screen and choose Use as Default Format from the Story Format top toolbar tab. A "Used as Default" sticker will appear on the card to confirm the change. If the Use As Default button is disabled, the format you've selected is already the default format, or the format you've selected is for proofing. (In that case, you probably want to set it as your proofing format instead.)
Changing the default story format doesn't affect stories you've already created, only new ones you create in the future. You can change the story format for existing stories in the Story Map screen.