How to update string in mysql

[tien nguyen]

Hi everybody. I have a string be save in mysql database. When i var_dump it like this.
string(280) "{"2016-01-07":{"available":"3","bind":"0","price":"1000000","promo":"0","status":"available"},"2016-01-08":{"available":"3","bind":"0","price":"1000000","promo":"0","status":"available"},"2016-01-09":{"available":"3","bind":"0","price":"1000000","promo":"0","status":"available"}}"
I want to update "available":"3" become "available":"2"
Note: number "3" is not a constants. It is a variables when I query from database.
Thanks you.
Sorry my english so bad.