how to adjust ken burns in a photos slideshow? 2022

You can't. The slideshow module is designed to be fully automatic/easy so doesn't give options here.

You might want to use iMovie to create sideshows with the ability to control ken burns.

There is not much to adjust. You can only adjust the total duration of the slideshow by clicking the clock item to the right of the slideshow window. The duration of the ken Burns effect cannot be changed.