In order to be able to void a purchase order you have to be given access to do this, if you do not already have access to amend a PO then please contact your project admin on your DPO project and they can adjust your user permissions for you (Admin > Users > Can Void)

Once you have access to void P.O's then click on the Purchase Order you would like to void. You will find the void button on the upper right hand side in the detailed P.O view.

After confirming your Purchase Order will be voided.

Note, this does not mean your P.O has been deleted. The P.O itself still exists and can be found in the filter of voided Purchase Orders.