replace a character

Howdy

how do I replace the a with a b in the image name and assign it to j ?

if i = "../../images/01a.jpg"

Thanks in advance

Code:

---

var original = "../../images/01a.jpg", j;
var replaceValue = "b";
if(original.indexOf("a.") !== -1){
        var j = original.split("a.")[0] + replaceValue + "." + original.split("a.")[1];
}
alert(j)

---

Thanks bud ;o))