Unfortunately you bought this as a kit. Otherwise I'd just tell you to return the remodel cans, pull out the soffit, and install new construction cans which would really be the best way to go.
Alternately drill four small holes thru the can just a hair above where the soffit will be. Install the can and then install four screws thru the holes you just drilled that will extend an inch or so over the soffit to hold the can in place. Essentially you are trying to pinch the soffit between the lip of the can and the screw.
Oh and bees seem to love building nests inside cans in a soffit so you are using an open trim watch you fingers when relamping in the summer.