replaceRange

Returns a char sequence with content of this char sequence where its part at the given range is replaced with the replacement char sequence.

fun CharSequence.replaceRange(startIndex: Int, endIndex: Int, replacement: CharSequence): CharSequence(source)

fun main() {
    val original = "Hello, World!"
    val modified = original.replaceRange(7, 12, "Kotlin")
    println(modified) // Hello, Kotlin!
}

Source