How to fix a slightly raised stair nosing board
If the top step is slightly higher than the hardwood flooring it shouldn't be much of a problem to pull it up and plane down the underside to the thickness you're seeking. You just don't want to overdo it. Be patient in removing the stair slat to prevent splitting.
EDIT
If the stair is mortised into the stringer the only realistic solution is to carefully use a sanding block to bevel the back edge of the step to reduce the risk. If you can keep the bevel area narrow and not impact the flooring you might be able to apply a matching stain and seal to the bevel that will be acceptable visually.