You can kill yarrow with any good lawn weed killer like the Bonide Weed Beater Ultra that we sell.  It is too hot right now to apply it (late July).  Wait until mid August when it gets below 80° in the daytime before you use the weed killer.  If you spray during hot, dry weather, the weed killer may kill the grass as well.  Also make sure that the lawn is well hydrated before you spray since a dry lawn can also be damaged by weed killer.  Once you do spray make sure it doesn’t rain within 24 hours of application and don’t water the lawn for at least 24 hours after.  Read and follow all the directions on the product’s packaging.