Over time, the resin will become degraded due to chlorine or lost due to natural attrition and must be replaced in order to maintain soft water. Some symptoms of resin needing to be replaced are:
Lack of water pressure. If placing the system on bypass brings the water pressure back up to normal pressure, the resin needs replacing.
Not maintaining soft water. If the system does not maintain soft water until the next scheduled regeneration could indicate the resin is no longer to the proper volume. Simply adding new resin to old resin is not recommended.

Place system on bypass and relieve the pressure. Disconnect the system from the bypass. Move system to a work area. Unscrew the valve head from the tank. The distributor tube may be difficult to remove. Using a garden hose, pinch the hose to control the water flow. Place the end of the hose on the end of the distributor tube. Using bursts of water from the hose, pull up on the distributor tube. Once the distributor tube is out, place tank between your legs and slowly lower tank to the ground to empty the water. Once the water is out, pick up tank and dump the resin into a bag or bucket or wherever you want to dispose of it. Use water to flush out all the resin from the tank.

To refill the tank, place the distributor tube back in the tank. Insure the top opening of the distributor tube is covered so no resin goes inside the tube. Pour in the under bedding (gravel) first then the resin. If the system does not require under bedding, you can fill with resin first then place the distributor tube in the tank. To move the tube through the resin, use the garden hose with short bursts of water. This will move the resin allowing the tube to fall into place.