Hi WLID1966,
As far as I know, you cannot update the Priority field from the PDP. What you can do is open your project through Project Professional and update the value.
Method to do it :
1. Open the Project in question using Project Pro.
2. Click on Project Tab and select Project Information.
3. Update the Priority field there.
Another option is to write a custom event, where you can create a custom field (call it Project Priority) and this can be used by the user to update the Priority field on Save through SQL.
For SQL you will find the Priority field as :
SELECT TaskName, TaskPriority
FROM [ProjectServer_Reporting].[dbo].[MSP_EpmTask_UserView] where TaskIsProjectSummary = 1