Holes can be filled with soil and overseeded. High spots are more problematic and might be able to be rolled out with a heavy roller. This would be easier if you rented a core aerator (a machine that removes a soil plug). That way when you roll the lawn it doesn't compress it too badly. The cores can also be used to fill in low spots.
If the lawn is extremely uneven a complete restoration may be in order i.e. killing the lawn, tilling raking, grading and seeding or sod. Less drastic, you could deal with it by top-dressing the lawn and over seeding.