sorry to hear that! it is quite impossible to "fix" severely overexposed video and the final image will never come close to video that was properly exposed to begin with. when I do such things for clients the term "salvage" gets thrown around... a lot of tutorials out there depending on what software you will be using, since they all offer different tools for correction. it is far easier to brighten underexposed images.
now, if by "fix" you mean "make sure video I shoot in the future is properly exposed" I'd recommend starting out by letting the x-star shoot in full auto mode. it does a decent job and you should be able to avoid severely over or underexposed images this way. when you watch the videos back, pay attention to if you would have preferred the video to be brighter or darker in particular places. next, with the x-star on the ground, on and pointed at a bright object, spend some time familiarizing yourself with the upper right wheel on the controller which will move your exposure value up and down, but still retain the ability to automatically lighten and darken. once you're comfortable with that you could play with full manual mode for ISO and shutter speed control. if you look for "ATTI Creek practice" video I did, that was on full manual otherwise the sun shining through the trees into the lens would have caused the x-star to reduce the exposure too much and detail would be lost in the shadows. just an example where you might prefer full control.
further down the road I think most of us use some sort of ND filter to keep the shutter speed lower for shooting smoother video and reduce prop shadows, not necessary for photos.
far better to take the time to get it right in camera then spend hours in front of the computer later.... just 2 cents from a video editor who has lost many hours of his life to the partial-salvage of bad footage placed in front of him best of luck!
You could try to use a color grading systems like DaVinci Resolve and load up one of the free luts to give it a more natural look but like Brian said you could be spending a lot of time in front of your computer correcting the video.