How to replace an uploaded file?

I am able to incorporate a file upload into a form OK, but if the user wishes to replace that file with a different one, the process runs into an error. Users can access the content they entered in a form via the dashboard widget. When Edit mode is activated, they can click on an "x" button to delete a file, which is then replaced by a Browse button.

However, even if they browse a new file and then Update, the file is not replaced. I suspect this is not the correct method to do this. What can I do?

Ah! I forgot that I am using the Gravity Forms Update Post plugin: http://wordpress.org/extend/plugins/gravity-forms-update-post/

Perhaps I should contact this developer. This is great feature for websites that are using Post fields as an easy way to publish content without access to the dashboard. I am using this method for a dirt-simple college e-portfolio system. The post revision method (which relates to my original question) is a necessary feature if we use the "form as publish" method.

... Also, I am granting Editors access to their form entries via dashboard widget with an added function in functions.php:

// Giving Editors Access to Gravity Forms
function add_grav_forms(){
	$role = get_role('editor');
	$role->add_cap('gform_full_access');
}
add_action('admin_init','add_grav_forms');