Saving values from input array wp widget plugin

I have a question about how to store/save input arrays on function `function update($instance, $old_instance) {
}

My input field looks like this

<input type=”text” id=”image_url” name=”image_url[]” value=”” />

Normally you save it like this

$instance[‘image_url’] = $new_instance[‘image_url’];
`
But how do i save it if i want to make use of input array?