setRange

Replaces characters in the specified range of this string builder with characters in the specified string value and returns this instance.

@IgnorableReturnValueexpect fun StringBuilder.setRange(startIndex: Int, endIndex: Int, value: String): StringBuilder(source)

fun main() {
    val sb = StringBuilder("Hello, World!")
    sb.setRange(7, 12, "Kotlin")
    println(sb) // Output: Hello, Kotlin!
}

Source