Bt is the answer. In a watering can, hose end sprayer, or sprinkle granules in the area.
https://www.dirtdoctor.com/garden/Armyworm_vq437.htmWhat is Bt? Bacillus thuringiensis is a biological treatment that stops the worms from digesting what they eat and they die.
https://www.dirtdoctor.com/garden/Bacillus-thuringiensis-Bt-Update_vq3307.htm Research shows that it must be used carefully, if you broadcast, for example, you'll kill butterflies. For your lawn, contact with the worms, and getting it onto the blades they are eating, is going to be your most effective treatment.