JSFiddle - React, Tailwind, and code Playground

JavaScript

function replaceAtPosition(number,index,replacement){
    number += '';//to string
    number = number.substring(0,index) + replacement + number.substring(index+1);
    return +number; // to int
}


var number = 101137111;
var newNumber = replaceAtPosition(number,1,9);

alert(number + ' => ' + newNumber);