in the IBAction for your button, try something like this:
Code:
NSString * appStoreLink = @"http://phobos.apple.com/WebObjects/MZStore.woa/wa/viewSoftware?id=361690226&mt=8";
[[UIApplication sharedApplication] openURL:[NSURL URLWithString:appStoreLink]];
You will need to replace the " id=361690226 " with
your app ID which you can find in iTunes Connect in your app description.