Hi linhsubeo,
Thanks for the question. I understand you are having issues downloading an application from the App Store. To troubleshoot these issue, let’s follow the steps for iTunes Store connectivity:
First, try signing out of your account in Settings > iTunes & App Store. Once signed out, go ahead and restart your device. Once your device has restarted, sign back in to the iTunes Store and try to download the update.
iOS: Changing the signed-in iTunes Store Apple ID account
http://support.apple.com/kb/HT1311
If the above did not resolve your issue, refer to the following:
Can't connect to the iTunes Store
http://support.apple.com/kb/TS1368
Thanks,
Matt M.