The first time a user launches an iOS app using push notifications they are presented with an alert view asking them whether or not they want to receive them.
If the user selects no, the app does not show under the notification center in the system settings menu so they can not change their decision there. If they select yes when shown the alert view the app is visible in the settings and they can toggle notifications on and off at will.
If the user selects no, is there any way you can give them the option to turn push notifications on from within the app itself?
No , it is not possible , you have to go to setting for that .
in version below ios 5.0 , it is possible to open setting page from your app , by
but in newer version it is not possible.