If you find you’re getting errors when using iTunes to update your iPhone, give the Settings app a shot. There are two ways to update an iOS device: using iTunes or within the Settings app. If iTunes Doesn’t Work, Try The Settings App (and Vice-Versa)
Now that the iOS update is deleted, go to Settings -> General -> Software Update to download the update again. If you see the update file, tap on it, then tap Delete Update. Scroll down the list of apps below the Storage Recommendations and look for an iOS update.
Occasionally, an issue with the downloaded iOS update file will prevent you from installing it on your iPhone.
While in Settings -> General -> iPhone Storage, check to see if the update has already downloaded. If you have more than 1 GB (gigabyte) available, then you have enough storage space to update your iPhone.