If the rubber tree plant is losing its leaves due to overwatering, cease any more watering until the top inch of soil is dry. If the soil has a damp, musty smell, it is likely to be suffering from root rot and requires further attention.
When plants are experiencing root rot, they require removal from the original pot, cutting away the black, mushy roots with scissors and repotting in fresh soil. Leave any healthy white roots in place.
If only the bottom leaves are turning brown and falling off the plant, it is probably short on nutrients. Most of the time, the missing nutrient is nitrogen. A loss of leaves could also indicate it is time to repot the plant. Because the roots are contained in the pot, they have a limited amount of room to grow and eventually begin to circle inside the pot. Roots sometimes start growing out the drain hole, eventually preventing it from absorbing needed nutrients and killing the plant.
While overwatering is a common cause of problems, not providing enough water also causes issues with rubber trees. The trees store water in their leaves like succulents, and waiting for the soil to dry encourages use of this water and healthy foliage. Eventually, however, the leaves become so dehydrated that they wither and die.