replaceCharacterString.swift

extension String {
    subscript(index : Int) -> Character{
        get {
            return self[advance(self.startIndex, index)]
        } set(newValue) {
            self = self.stringByReplacingCharactersInRange(Range<String.Index>(start: advance(startIndex, index), end: advance(startIndex, index+1)), withString: String(newValue))
        }
    }
    
}

var string = "Hello World!"
string[4] //OUTPUT -> "o"
string[4] = "y" //OUTPUT -> "Helly World!"