The system will only allow you to change the approval type if there are no orders in the status awaiting approval, if there is you will see a warning message when trying the save the changes.
Steps to change the approval type:
Click the Admin Cogs along the top bar.
Click Mini Markets, then Manage Mini Markets.
Edit the mini market, and stay on the Details tab.
Scroll down and open the sub section Approval Options.
Select the new approval type from the drop-down box and click Save Settings at the bottom of the page.
To learn more about regarding the options please visit our article covering the different approval types.