Hi Kendra,
My name is Jocelyn, an independent advisor here to help.
I understand that you would like to switch back to S mode but once you switch out of S-mode you cannot go back unless you will reinstall your Windows.
However, in normal you can set your computer to only allow apps from the store to increase security and use Microsoft Edge for browsing to keep your computer safe.
Here's how to allow your computer to only download from the Microsoft Store.
Go to settings > apps > apps & features "choose where to get apps"
From this article Windows 10 in S mode FAQ
https://support.microsoft.com/en-ph/help/402008...
Switching out of S mode is one-way. If you switch out of S mode, you'll need to keep using the standard version of Windows 10.
I hope this information help. Let me know if you need anything else.
Kind Regards,
Jocelyn