Do you mean the bothie mode? If not then you can just use the FFC to take a selfie.
The selfie screen behaves like a mirror for easier handling before taking the picture, but the picture is stored the (proper) way other people see you.
There is currently no option to change this behaviour in HMD's camera app on Nokia 8.
Install an editor like Google's Snapseed to flip the selfie if you must, and do lots more to both selfies and other pictures. It's a bit nerdy but mature and reliable once you get used to it.