Assuming it hasn't been touched in a number of years, it doesn't look all that bad.

In theory you'd pull it all and redo the base to the proper level and relay the brick. But this is an old walk, and that's a lot of work. Its rarely used and most of the settling has probably already happened. If you are ok with a 7-10 year job, then I'd top up the low spots and relay the brick. I redid a paver patio this way, it held up plenty well until we moved like 7-8years later (we were on really well draining soil - which likely played a part of it.)