how to replace certain characters in string swift

var oldStr ="Hello";
var newStr = oldStr.substring(1); //remove first character "ello"

let aString = "This is my string"
let newString = aString.replacingOccurrences(of: " ", with: "+", options: .literal, range: nil)

extension String {
	func withReplacedCharacters(_ oldChar: String, by newChar: String) -> String {
	    let newStr = self.replacingOccurrences(of: oldChar, with: newChar, options: .literal, range: nil)
	    return newStr
	}
}

String words = "Hi There"; //creating a string var named 'words'
char index = words.charAt(0); /* setting a variable 'index' to letter
'0' of variable 'words'. In this case, index = 'H'.*/
System.out.println(index); //print var 'index' which will print "H"