To re-order the days on the campaign, click the edit icon on the day and change the day # in order to change the order of your steps. You have to always have a day 1, but otherwise can switch them as you like.
Warning: be careful not to do the re-ordering while you have campaigns in flight. If you change the order it will affect all campaigns, which means some emails could potentially not be sent in the correct order. Be sure to remove everyone from the campaign before changing the order