Remove the filter from the apparatus on which it is attached.
Fill a bowl or bucket with one part white vinegar and one part water. If you prefer, you can use a commercial cleaner made specifically for salt. Follow the instructions on the cleaner when mixing your solution. Make sure there is enough of the solution to cover the filter.
Soak the filter for about an hour. Rinse it in plain water and wipe dry. The salt resin should be gone; if it isn't, soak it again for another hour.
Wipe off any remaining resin with plain vinegar on a rag.