From http://www.w3schools.com (Copyright Refsnes Data)

WMLScript replaceAt() Function

The replaceAt() function divides a string into elements and replaces a specified element.

n = String.replaceAt(string, substring, index, separator)

Part	Description
n	The string returned from the function
string	The original string
substring	The substring to be replaced with a specified element in the original string
index	An integer that specifies where to replace the substring
separator	The dividing separator

var a = String.replaceAt("Visit W3Schools!","I love",0," ");
var b = String.replaceAt("Visit W3Schools!","us!",2," ");

a = "I love W3Schools!"
b = "Visit us!"