If the date you want to change is for December 22, 2010 ... and you want to set it to December 25, 2010 then you could use:
Code:
UPDATE products SET products_date_available = '2010-12-25' WHERE products_date_available >= '2010-12-22' and products_date_available < '2010-12-23';
Before doing this, you want to make a backup of your database first ...