Since you have the control valve off, lean the resin tan over and pour out all the water you can get out without resin. Then mix a 1/2-3/4 cup of Iron Out/Super Iron Out in enough water to dissolve all of it and pour that in the resin tank and fill the tank with fresh water. Install the control valve. Let it sit for a few hours and then add 2 gallons of water to the salt tank, not down through the salt, wait 2 hours and do a manual regeneration.
The only other way is to empty the tank and let it dry out inside and then use a brush on a stick to scrape the rust off the inside of the tank. Do not let the resin dry out, keep it covered with water at all times.