my MacBook air is stuck during download for OS update, help

Many times this is caused by issues with using a wireless connection. At this point, you basically have three options:

1. Keep trying to download using a wireless connection.
2. Consider getting a Thunderbolt-to-Gigabit Ethernet adapter for just these occasions.
3. Pay a visit to your local Apple Store or Apple Authorized Service Provider to assist you with this.

Are you seeing an error message or does the download process just "hang" somewhere after starting it? Is the notebook connected to the network by wireless or by wire? If the former, have you tried downloading while it is connected by Ethernet?