1) I want my application to be updated without showing anything to user so I want to hide all windows which are showing during updates. Is this possible in wybuild? I am using AutomaticUpdaterBackend for auto update.
Yes, but only if you make a "LoaclSystem" Windows Service and use the AutomaticUpdaterBackend from that. But since you're using a GUI app, it is highly recommended that you leave the UI.
2) In window7 when updates are available, UAC prompt is shown for permissions. Can i suppress this prompt anyhow during installation?
If you install your app to the %appdata% folder (and you don't update any system files), then wyUpdate won't prompt for elevation. Or if you run wyUpdate from a LocalSystem Windows Service you won't need to elevate.
3) Also I want to change text in the title bar of form. Project which I am doing needs branding of client company so I cannot use wyupdate in text. Can I do this?
Yes. See File -> Properties -> wyUpdate. You can change the title bar text there.