Do I have to enter Username and Password each time I want to update the system ?

No, once they are listed in the control center and saved - you should not have to re-enter them again. If you do not allow the saving of passwords on your computer then you will need to enter them each time you update and you will need to set update to "ask". Since these Username Password are only used for access to our server and could not expose any of your personal information - we suggest you allow them to be saved in the control center so that automatic updates are possible.