Answer: If you think you have overwatered, cut back slightly on the watering. Let it dry slightly between waterings, do not keep it sopping wet all the time. If you use a saucer under the plant, empty it of excess water after watering.
Also maintain high humidity surrounding the plant, possibly by setting it on a tray filled with pebbles and water. (The bottom of the pot should be just above the water level, it should not sit in the water.)
If temperatures have been very high, it may be reacting to that. Try to keep it somewhat cool with a maximum temperature of 75 being ideal.
Sometimes a plant will live on its reserves and tolerate low light for a while but eventually shows stress. This plant actually does well in medium but indirect light, so you might try gradually increasing the amount of light it receives.
Another possibility, if you have been fertilizing it heavily, is that salts have built up in the soil. If you think this is possible, water thoroughly so water runs out the bottom of the pot, wait a little and water heavily again. This should leach the salts out of the soil. Then return to normal watering, letting it dry a bit in between watering. These can be fertilized lightly from spring to fall, you could use a water soluble fertilizer for foliage plants (such as 10-10-10 plus minors at the weaker rate on the label.)
It might also be a case of insect infestation. Ferns are sometimes affected by mealy bugs (look like bits of white cotton, dab with a cotton swab dipped in rubbing alcohol) or scale. Scale insects look like dark discs on the stem or foliage. This is very difficult to control, but you can try gently rubbing them off and discarding the worst affected stems.
One other possibility although not too likely is that there may be spores forming, if what you are seeing is a regular pattern of dots or circles only on the underside of the fronds. This would not really be a problem per se.
In any case, if the foliage or stems are dried out and dead, trim them off and discard in the trash, also clean out any debris from the pot. This will help limit reinfection if it is a disease or pest.
Good luck with your fern!
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