I'm not sure which thing you're asking, so I'm going for two answers:
1: There's a little arrow thing at the top of the camera

that switches the bottom menu from Video Photo Portrait to other stuff, including the aspect ratio. This can be set to 4:3 or 16:9 or Square. 4:3 is native, it's the actual aspect ratio of the sensor, so I keep it there, and I do any cropping later.
2: Some adjustments you do to the camera will spring back to the default when you close the camera app. If you'd like to keep one set so it doesn't undo what you did, then you can go to Settings>Camera and hunt for Preserve Settings>Creative Controls --
turning that on will keep it from changing until you tell it.