Add more topsoil to your lawn to even out areas with less than an inch of difference. This amount of soil shouldn't kill the grass beneath it, which will eventually grow over the topsoil you added.
Shovel on more topsoil in areas with a depth greater than 1 inch. Since this amount of topsoil is more likely to kill the grass beneath, you'll need to reseed.
Add grass seeds to the topsoil that needs to be reseeded. Rake over the seeds once they're added to the soil.
Water the seeds thoroughly once they're laid to promote growth. Continue to water daily until the grass starts to sprout.