Q: I can't download application.it's require some updates form.how can I update my iPod

Connect the iPod to your computer's iTunes and copy any purchases off the iPod to your computer via File > Devices > Transfer Purchases. You may also want to copy photos and any important documents off the iPad as well e.g. via the file sharing section at the bottom of the device's apps tab when connected to iTunes, via wifi, email, dropbox etc - they should be included in the backup, but it's best to have a copy of them outside of the backup just in case. You can then force a backup of the iPod via File > Devices > Back Up

Then start the update by clicking the Check For Updates buttons on the device's Summary tab

Updating to iOS 5+ : http://support.apple.com/kb/HT4972

Your iPod will be updated to the highest iOS version that it supports