software update

Hi I would recommend you to try these steps to update your iPhone:

On pc:

Update software on iPhone, iPad or iPod touch in iTunes on PC

Using iTunes, you can update software on your iPhone, iPad or iPod. 

1. Connect your device to your computer.
2. You can connect your device using a USB or USB-C cable or a Wi-Fi connection. To turn on Wi-Fi syncing, see Sync iTunes content on PC with devices on Wi-Fi.
3. In the iTunes app  on your PC, click the Device button near the top left of the iTunes window.
4. 
5. Click Summary.
6. 
7. Click Check for Update.
8. To install an available update, click Update.

obtained from: Update software on iPhone, iPad or iPod touch in iTunes on PC – Apple Support (UK)

On Mac:

Update software on your iPhone or iPad

On your Mac, you can update software on your iPhone, iPad or an iPod touch. 

1. Connect your device to your Mac.
2. You can connect your device using a USB or USB-C cable or using a Wi-Fi connection. See Sync content between your Mac and iPhone or iPad over Wi-Fi.
3. In the Finder  on your Mac, select the device in the Finder sidebar.
4. 
5. If you connect your device to your Mac using a USB cable and can’t see the device in the Finder sidebar, see If your device doesn’t appear in the sidebar. 
6. Click General in the button bar.
7. 
8. Click Check for Update.
9. 
10. To install an available update, click Update.

obtained from: Update software on your iPhone or iPad – Apple Support (UK)