How do I modify Purchase Order
 

The current purchase order payment option requires 4 fields to be filled in - how do I make only the PO Number required?

Re: How do I modify Purchase Order
 

You can do it from a custom module.

These are the files that control the form:
- \XLite\Model\Payment\Processor\PurchaseOrder class
- skins/default/en/checkout/purchase_order.tpl template

Most likely replacing the template file with a custom one that displays only a single PO field will be enough. You can do it either by making your module to be a custom theme, or by decorating the getInputTemplate() method in PurchaseOrder class and making it refer to your custom template file.