update 8.1 unable to download and install

Try Syncing with iTunes. You can also perform the upgrade that way. 🙂

Update the iOS software on your iPhone, iPad, and iPod touch

Update your device using iTunes

If you can’t update wirelessly, or if you want to update with iTunes, follow these steps:

1. Install the latest version of iTunes on your computer.
2. Plug in your device to your computer.
3. In iTunes, select your device.
4. In the Summary pane, click Check for Update.
5. Click Download and Update.

If you don't have enough free space to update using iTunes, you'll need to delete content manually from your device.

Some iOS software updates aren't available as over-the-air updates. VPN or proxy connections might prevent your device from contacting the iOS update servers.