string = "this is a string";//I need to replace index 0 to 3 whith another string Ex.:"that"
result = "that is a string";

result = string.replaceAt(0, 'that');

function replaceRange(s, start, end, substitute) {
    return s.substring(0, start) + substitute + s.substring(end);
}

var str = "this is a string";
var newString = replaceRange(str, 0, 4, "that"); // "that is a string"

var str = "this is a string";
var newString = str.substr(3,str.length);
var result = 'that'+newString

var str = "this is a string";
var firstPart = str.substr(0,7); // "this is "
var lastPart = str.substr(8,str.length); // " string"
var result = firstPart+'another'+lastPart; // "this is another string"

var str = "this is a string";
var result = "that" + str.substring(4);