You can update your phone to 15.7.5, if it doesn't work doing it wirelessly, then see the instructions below for how to update using a computer.
Update your iPhone, iPad, or iPod touch
• If you have an iPhone 5S or 6/6plus, the highest you can update to is 12.5.7
• If you have an iPhone 6S through 7, you can update to 15.7.5
• If you have an iPhone 8 or higher, you can update to 16.4.1(a)
Go to Settings > General > Software Update.
This link will show you how to update your iPhone, iPad, or iPod touch using WiFi -
Update your iPhone or iPad - Apple Support
This link will show you how to update using your computer -
Update iOS on iPhone - Apple Support