var myString:String = "hello";
trace(myString);
myString = catipaliseFirstLetter(myString);
trace(myString);

function catipaliseFirstLetter($s:String):String {
	return $s.substring(0, 1).toUpperCase() + $s.substr(1, $s.length-1);
}

// OUTPUT
// hello
// Hello

AS3 Capitalise the First Letter of a String

ActionScript 3