How to fix your iPhone
1. Full StorageYou've just gotten a notification for an upgrade but hit a wall while actually doing it because of storage. Annoying right?
You can find an alternative photo storage solution and delete old iCloud backups. This is because photos and videos are the biggest culprits but you can still check to know if there are others. Go to Settings > General > Storage > Manage Storage.
If it is photos, use Google Photos - the free app and website, to store unlimited photos. In the app's settings, you can tap "Free Up Space" to delete original photos and videos from your device once they are backed up.
If that does not work, check your iCloud account. It may be full of backup from past phones. Check Settings > iCloud > Storage > Manage Storage. Then tap the outdated backup to delete it.