By default WP doesn't allow downloads over 50 MB through mobile network. Which was later increased to 100 MB. This is done so as to keep a check because mobile data is comparatively expensive(much more in the US than here in India for ex).
In any case NSU can be used to download the firmware on your PC and update your phone. Please do keep in mind of the following things:
- If you download the firmware, the entire OS will be downloaded instead of just the update(only update is downloaded in OTA updates). The size will be upwards of 1.3 GB.
- You will lose all data if you update by connecting to PC. App lists, contacts, messages can be backed up. But you will lose all app data including game progresses. Xbox Achievements will be retained.
You can download NSU by
clicking here.