How to edit number with different steps in menu?

You need to change the value for all the items too, so:

var number = 50.0;

function onChange(v) {
  number = v;
  menu["number x10"].value = v;
  menu["number x1"].value = v;
  menu["number x0.1"].value = v;
}

var menu = {
  "" : { "title" : "-- Menu --" },
  "number x10" : {
    value : number,
    format: v => v.toFixed(1),
    step: 10,
    onchange : onChange
  },
  "number x1" : {
    value : number,
    format: v => v.toFixed(1),
    step: 1,
    onchange : onChange
  },
  "number x0.1" : {
    value : number,
    format: v => v.toFixed(1),
    step: 0.1,
    onchange : onChange
  },
  "Exit" : function() { E.showMenu(); },
};