Question
how do I kill lawn mushrooms?

Answer
Hi Marlene;
Mushrooms are fungus, so a fungicide, or if you are an organic gardener, baking soda disolved in water.
Mushrooms come from something in your soil decaying. Usually it will be  pieces of buried wood, like scraps the builders sometimes and just cover with a little soil they put on the site ( never eniough to do any good), or dead roots.
If it is an ongoing problem, and the mushrooms are in one area only, you will want to dig down and see if there are dead roots or pieces of wood buried there.
You can sprinkle baking soda on the ground and water  it to disolve it and soak it in, or disolve it in water in your garden sprayer.
If you are not on an organic program and use chemicals, Funginex would work.
I personally hink baking soda works better than Funginex.
I used to use it on my roses for black spot  and on my crepe myrtles for powdery mildew. It worked, but took repeated applications. Now I mix baking soda with water in my garden sprayer, and spray at the first new growth, and it completely keeps those diseases away.
Mix about 2 TBLSP baking soda per gallon of water.
Charlotte