Option I - If You Have iOS 14
To be able to do this, you'd need iOS 14 running on your iPhone which you can do by following this guide. Once you have the latest software up and running on the iPhone, simply tap the Settings app and scroll down to find the Camera app.
Tap the Camera app and toggle the switch next to Mirror Selfies. When enabled, your selfies would remain in the mirrored state and when you disable this toggle, the selfies would be saved as how it has always been, natural.
Option II - If You Don't Have iOS 14
Before iOS 14, if you wanted to have mirrored selfies then your only option was to manually flip the images using the inbuilt editor in the Photos app. If you want to flip the image after you've captured it, open the Photos app and tap the image you want to flip. Tap the Edit button on the top right corner and select the crop function from the bottom.
On the top left corner, you'd find the flip button, tap the flip button on the top left corner to mirror your Selfie and select Done.
Updated: October 10, 2021