If you accidentally take a video in the wrong orientation, the video may show up sideways on your frame, computer and phone. To fix this on a Mac you do the following
First, your video must on your Mac.
Second, you want to open the video with QuickTime.
Most likely your video will be on Apple Photos. Find the video and right click it to reveal a small side menu. The Side menu should contain Edit With. Click on it. Then click on QuickTime Player.
If your video isn't in Apple Photos but in your Mac's file system you can find the video in Finder and right click it. Then select Open With and choose QuickTime Player.
Finally, use QuickTime Player to rotate the video.
When QuickTime player opens up, go to its Edit Menu and you will see options to rotate the video left or right.
Once its rotated, go to the File Menu and select Share to. Then select Apple Photos. Or you can Export it as a 720p file and save it somewhere on your Mac.