Attach an automatic, chromatic tuner, take the lid off and look for the little tuning spring on each tine.
While monitoring the tuners display, just move the spring back and forth to find the exact pitch.
On the lower half of the keyboard, this can be done with your fingers, and the best way to do it on the upper half is using a small screwdriver.
It's not totally uncomplicated, but mostly it takes patience.
Good luck,
Freddan
