Changing aspect ratio from 16x9 to IG (9x16)

This is driving me crazy and I know there must be an easier way to do this, but can anyone show me how to take a regular clip shot and edited in 16x9 and change it so it can fit on IG (Instant Gram)? I know of course to make a copy first, but I’m having difficulty finding out how to accomplish this. I have no clue why programs like IG require you to film Portrait mode, but it is what it is and my client wants some of the interviews in that mode that I’ve already produced. Any help would be grateful. Warmly, Eric