Reset is extremely flexible and it's common for Reset members who have a set plan to swap days in a given week (e.g. Reset Day is normally Monday but switched to Tuesday due to an event on Monday).
Go to the Reset Day you want to change and click the "Lightning Bolt" icon. You will then see the option to "Switch to Free Day." Likewise, you can go to any Free Day and click the icon that says "Switch to Reset Day."