The 'bar' method can be called to get the progressbar object, and then call 'setValue' method for this progressbar object to set a new progress value.
$.messager.progress({
title: 'Please waiting',
msg: 'Processing...',
interval: 0 // disable auto update progress value
});
var bar = $.messager.progress('bar'); // get the progressbar object
bar.progressbar('setValue', 60); // set new progress value