Need help changing ratio on iphone video shot verticle because iMovie is showing it stretched wide.

What do I do if I imported a video from my iphone to imovie and it came out stretched? Like it was shot with camera upright and the imovie is making it fit to frame. I don't mind if it plays with two black bands on the side I just don't want it stretched.

I don't se anywhere to adjust aspect ratio or anything that seems to help and most of the answers in forums use tech talk I don't understand. So if you are going to suggest I change the ratio from 16:9 to 16:10 or something like that please explain what that is and how I specifically do it in iMovie for Dummies talk.

Thanks so much.