Because yellowing and falling leaves can be a sign of over or underwatering, see if the soil feels dry. If the soil is damp, chances are your plant is experiencing root rot from overwatering. Remove the plant from its pot and let the bare roots air-dry for 24 hours.  Repot into a container with drainage holes, filled with a well-draining soil mix. Make sure the container is only an inch or so larger than the root ball. Put the plant in indirect sunlight and water sparingly. If the soil is very dry, you may be able to revive the peperomia rotundifolia by setting the pot in a deep dish of water for 10 minutes. This allows it to absorb water from the bottom up; if successful, the leaves should become plump and firm again within a few hours.