How do I reinstall iOS without losing data?

iOS: Enable Airplane mode and disable Wi-Fi and Bluetooth in Settings. iTunes: Create an encrypted backup of your iPhone and verify that the process completed successfully. iTunes: Select your iPhone and press “Restore...”. This will download the current iOS 11 release and install a fresh copy on your iPhone.

Will my photos be deleted if I restore my iPhone?

The photos and any other data you had yet to backup to iTunes will be lost. Because it is a restore of the data in the iTunes backup, your current data including photos on your iPhone will be erased and replaced by the photos in your backup.