The best thing you can do for overwatered plants is give them time between waterings, and then start off watering slowly until things seem back to normal. Make sure that water is able to drain easily out the bottom of potted cannabis plants. Be extra careful with small plants in big containers.

How to Water Cannabis Properly:
Wait until the top of the growing medium is dry about an inch deep (up to your first knuckle).
Add water until you see some at least 20% extra runoff water drain out the bottom of your pot. Go back to step 1.
If top of growing medium stays wet for a long time, you may need to give your plants less water at a time, or improve your drainage.

NOTE: The goal is to be watering your plants every 2-3 days. If it needs longer to dry out, you should be giving less water at a time. If it’s drying out too quickly it should get more water at a time (or may need to move to a bigger pot).