Hello there jojojimenez,
It sounds like you have confirmed you have fewer photos in the Photos app than are actually being reported in the Settings. I would use recovery mode to update the iOS. It shouldn't compromise the data that you have on the device, but creating a backup of your phone first is always a good idea, either to iTunes or iCloud:
Should I use iTunes or iCloud to back up my iPhone, iPad, or iPod touch?
and to do the update on your device use this section of the following article:
If your iPhone, iPad, or iPod touch doesn't respond or doesn't turn on
- Make sure that you have iTunes 12 or later on your computer.
- Put your device in recovery mode.
- When you get the option to restore or update, select Update. This will reinstall iOS without erasing your data.
Thank you for using Apple Support Communities.
Take care,
Sterling