S9 Anyway to rotate a video in Samsung video player or editor?

[Android Central Question]

I recorded a video the other day holding the phone horizontally and upright, but when I play it back it will only fill the screen if I hold the camera vertically? First of all, what the hell? And secondly, is there an easy way to rotate a video without downloading new software?

p.s. I think I know why, it's because I started the video with the camera facing the ground.