In article ers.com,
"Peter H" wrote:

You could roll it, although this method is frowned upon to some degree by
the pros these days or you could topdress it w/ topsoil or tripplemix.

The back yard is fairly new at 4-ish years old. I know it wasn't this
bumpy when I seeded - I rolled it as prep for the seed. The bumps are
hard enough that I don't think rolling will work to smooth it out. I did
consider topping but am unsure how to smooth that - unless you mean to
completely submerge the existing lawn under the new soil.