How to replace nth character of a string with another

How could I replace nth character of a String with another one?

func replace(myString:String, index:Int, newCharac:Character) -> String {
    // Write correct code here
    return modifiedString
}

For example, replace("House", 2, "r") should be equal to "Horse".

Please see NateCook answer for more details

func replace(myString: String, _ index: Int, _ newChar: Character) -> String {
    var chars = Array(myString.characters)     // gets an array of characters
    chars[index] = newChar
    let modifiedString = String(chars)
    return modifiedString
}

For Swift 5

func replace(myString: String, _ index: Int, _ newChar: Character) -> String {
    var chars = Array(myString)     // gets an array of characters
    chars[index] = newChar
    let modifiedString = String(chars)
    return modifiedString
}

replace("House", 2, "r")

This is no longer valid and deprecated.

You can always use swift String with NSString.So you can call NSString function on swift String. By old stringByReplacingCharactersInRange: you can do like this

var st :String = "House"
let abc = st.bridgeToObjectiveC().stringByReplacingCharactersInRange(NSMakeRange(2,1), withString:"r") //Will give Horse