If you don't need all of the features of TurboTax Self Employed, you can downgrade from TurboTax Self Employed to another version of TurboTax Online, as long as you haven't paid yet (or registered in a free version) - but please bear in mind that you will need to clear any data which you have entered, and start over in the other TurboTax version. This is the only available method to switch versions.
Please see the following TurboTax FAQ, "Can I switch to a lower-priced version of TurboTax Online?":
To file a return for a S corporation (Form 1120-S), you'll need TurboTax Business, which supports Form 1120S (U.S. Income Tax Return for an S Corporation). TurboTax Business is a Windows-only software program available as a CD or download.
The following TurboTax FAQ, "Where do I find Form 1120S for an S Corporation?", contains a link to the TurboTax Business product: