This may work:
Connect iPhone and your computer with a cable - Apple Support
..If your iPhone came with a USB-C to Lightning Cable and your computer has a USB port, use a Lightning to USB Cable (sold separately). The first time you connect the devices, select Trust when asked on iPhone whether to trust this computer. (See the Apple Support article About the 'Trust This Computer' alert .) Do any of the following..
Update iOS on iPhone - Apple Support
Go to Settings > General > Software Update. The screen shows the currently installed version of iOS and whether an update is available. To turn off automatic updates, go to Settings > General > Software Update > Automatic Updates. Update using your computer Connect iPhone and your computer with a cable. Do one of the following
When you haven't a PC or Mac, an idea would be use public library. Or contact an AASP or Apple store genius;
and arrange for help to check the phone, and see if it needs extra steps. Service charges may apply.