You can use the Availability Sets page to edit the description of an Availability Set resource for the target Azure platform.
In the Web Interface toolbar, select the Platforms tab.
Select the platform, then click Edit.
Under Associations, select the Availability Sets tab, select the set of interest, then click Edit (or double-click the availability set) to open the Edit Availability Set dialog.
Modify the Description for the Availability Set resource.
Click Save to update the Availability Set resource.
Click Close to exit the dialog.