You certainly can reduce the speed of the props on a mini1 or
mini2: reduce altitude as fast as possible, and/or land the drone. That causes the motors/props to spin slower and develop less thrust. I suspect that's not what you want though.
If what you're after is a lower prop speed while still maintaining thrust then the only option is to change the props for a set that produce more thrust for a given prop speed.
Master Airscrew produce some aftermarket props for the
Mini2 that spin slightly slower than the DJI props, going by the tone of the prop noise.