While new iPhones can take 4K videos, the older models (like iPhone 5) are limited to Full HD recording. To change recording resolution and video quality on iPhone, do as follows:
1. Go to the Settings from the home screen
2. Scroll down to Photos & Media and open it (You may find it below the Music and Video settings)
3. Scroll all the way down to the Record Video option under the Camera settings
4. Open the Record Video option and change video quality on iPhone from the available resolution and fps options.