1323-maximum-69-number.js

/**
 * @param {number} num
 * @return {number}
 */
var maximum69Number  = function(num) {
  const arr = (num+'').split('')
  for(let i = 0; i < arr.length; i++) {
    if(arr[i] === '6') {
      arr[i] = '9'
      break
    }
  }
  return arr.join('')
};