transition it with the adult food just like making a food change... Also they will eat it if you put it down... They might be stubborn but eventually they get the hint this or nothing
I've compared the ingredients of over 20 brands of cat and kitten foods on a spread sheet once as a project.
Something I found interesting was that many kitten foods were the same nutritionally as their adult food of the same brand. The difference was in the size of the kibble.
I agree with what has been said here, Kitten food is only good for kittens, or pregnant/nursing momma cats, otherwise your adult kitties are just going to gain weight. Mix the kitten and adult food until there is no more kitten food in the bowl.