Samsung Software Update - How to fix or Undo

A

AC Question

I have Samsung S6 Edge +.... software update came on my screen this morning and I allowed it to take place. I seem to recall seeing the word Marshmallow.... so I figure it's the operating system going from Lollipop to Marshmallow.
Well... now I can't see any sms messages, I can't create a message, I can't make phone calls. I can hear a call coming in but there is no way to answer the call - it's a blank screen when a call comes in. Something's very wrong. How do I fix or undo this update please ?!?!
Melanie