OCaml Language

let num [ 1; 2; 3 D:; What is the command to retrieve the second value from num? Since variables are not mutable, that is, the value in them can not be changed, how would I make a variable, num, if I wanted to store 5 in it, but make it so that I could change the value that is in it after storing 5 in it? That is to say is there a way to make it mutable, and if so, what command would I need to use to do that? Give an example of a for loop. Give an example of a while loop