I do not have a Mantis Q but from what little I have seen of the screen from the controller it looks very similar to our Typhoon H Plus. Below the bright red 'Record' button you will see an icon that looks like three sliders. Tap this and you will go into 'Camera Settings'. From there look for a heading called 'Video Format' - tapping this button will most likely get you to where you can change the resolution as well as others items like FPS and so on. If not hopefully someone can come along and help you out more.
This is a video about all of the controls to an H Plus, if you skip to the 4:00 mark you will see the steps I describe above. Good Luck.