I efiled last month and it was rejected. I went on today to fix the return and accidentally hit "premier" How do I downgrade?

The only way to downgrade is to Clear and Start over.  To do so, see the following help article: How do I clear and start over in TurboTax Online?

I already filed and it was rejected. So clearing isn't an option. And "removing" isn't one either. 

You can proceed in Premier, or you can contact customer support:  Contact us