Please try the following:
1. Try connecting to a network with a better signal. An unstable network may cause the downloaded upgrade package to be damaged or missing.
2. Ensure your phone has enough storage space, so the download of the installation package can be completed.
3. Ensure your phone's has more than 40% power — less won't allow OTA upgrades. Upgrade after charging your phone for some time.
4. Avoid using your mobile phone during the OTA upgrade, so you don't interrupt the upgrade. If you've used it during the upgrade, perform the upgrade again.
If the above methods do not resolve your issue, please contact the Nothing customer service team for further assistance.