mapKeys

Returns a new Map with entries having the keys obtained by applying the transform function to each entry in this Map and the values of this map.

inline fun <K, V, R> Map<out K, V>.mapKeys(transform: (Map.Entry<K, V>) -> R): Map<R, V>(source)

fun main() {
    val original = mapOf("Apple" to 3, "Banana" to 5, "Cherry" to 2)

    // Convert all keys to lower case
    val lowerCaseKeys = original.mapKeys { entry -> entry.key.lowercase() }

    println(lowerCaseKeys)   // {apple=3, banana=5, cherry=2}
}

Source