Update PaperCut NG over the top of your existing licensed version. Customers with current Upgrade Assurance are licensed to upgrade at any time. A one-off purchase may also be upgraded to a minor point release or bug fix within the same version (e.g. 18.0 to 18.1). An upgrade may be purchased from the About tab of the admin interface if Upgrade Assurance has expired.
For customers using Online Payment Gateways, when upgrading to PaperCut NG 18.0 from a version previous to 16.2, you must also upgrade the Payment Gateway module. For more information see our Upgrading Knowledge Base article.