BACKUP DBASE FIRST!!!!!

UPDATE `zen_products` SET `products_image` = REPLACE(`products_image`, "JPG", "jpg") WHERE RIGHT(`products_image`, 3) = "JPG";

Naturally, if your dbase does NOT use prefixes...

UPDATE `products` SET `products_image` = REPLACE(`products_image`, "JPG", "jpg") WHERE RIGHT(`products_image`, 3) = "JPG";