As long as you have an active Gliffy license, you can update the Plugin any time from the Universal Plugin Manager (UPM):
1. Open the Confluence administration menu and select Add-ons (must have Admin rights).
2. Log in with your Confluence Admin credentials.
3. In the left menu, select Manage Add-ons to open your Universal Plugin Manager (UPM).
4. Use the drop-down menu at the top to select Action Required add-ons.
5. If there is an available update for Gliffy, click on Gliffy Diagrams for Confluence then click the Update button.
6. The update should show a “Success!” screen if installed successfully.