toMutableList

fun CharSequence.toMutableList(): MutableList<Char>(source)

fun main() {
    val original = "Hello"
    val mutableList = original.toMutableList() // ['H', 'e', 'l', 'l', 'o']

    mutableList[1] = 'a' // change 'e' to 'a'
    println(mutableList.joinToString("")) // prints "Hallo"
}