Use the setAttribute
method:
document.getElementById('item1').setAttribute('data', "icon: 'base2.gif', url: 'output.htm', target: 'AccessPage', output: '1'");
But you really should be using data followed with a dash and with its property, like:
<li ... data-icon="base.gif" ...>
And to do it in JS use the dataset
property:
document.getElementById('item1').dataset.icon = "base.gif";