You can run a core aerator over it (or do it manually with a hand aerator depending on size) - you want one that cuts out small plugs, not one that cuts slits. Once you have pulled up a bunch of cores and let them dry on the surface, you can rake those around a bit and fill in some low spots, but for serious lumps you may need to bring in material to get towards level. If you do that gradually, the grass will continue to grow, where if you try to get it all in one go some will be so deeply buried you'll have to re-seed.
Adding 1/4" of compost will help with both health and texture.
While you are describing lumps rather than depressions, my answer here should still apply for the gradual solution approach: How should I fill and reseed shallow depressions in my lawn?