UPN diagrams can be drawn with any software that supports connected lines and boxes, for example office suites, and specialist charting tools such as Visio and Lucid chart. But for managed process repositories you need a UPN centric application.
There are a few to choose from. Some have free trials, one has perpetual free access with limited functionality. Click the logos for more information. If you know of any more please message me using the Contact link above.
WordPress + DrawIt plugin will give you a very rudimentary CMS + UPN diagram capability