Update PaperCut NG over the top of your existing licensed version. Customers with current Maintenance & Support 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. 21.0 to 21.1). An upgrade may be purchased from the About tab of the admin interface if Maintenance & Support has expired.
For customers using Online Payment Gateways, when upgrading to PaperCut NG 21.0 from a version previous to 16.2, you must also upgrade the Payment Gateway module. For more information see our Upgrading Knowledge Base article.