How to fix without a desktop computer

Update your iPhone, iPad, or iPod touch - Apple Support

Update your device wirelessly. If a message says that an update is available, tap Install Now. 

These are the iOS options based on iPhone Model --> Learn how to--> Update your iPhone, iPad, or iPod touch - Apple Support

• iOS 13.5.1 —> 6S / 6S Plus, SE, 7 / 7 Plus, 8 / 8 Plus, X, XR, XS /XS Max, 11,11 Pro /11 Pro Max
• iOS 12.4.7 —> Final release supported on iPhone 5S, iPhone 6/6 Plus, iPad Air, iPad Mini 2, iPad Mini 3 and iPod Touch (6th generation)
• iOS 10.3.4 —> Final release supported on iPhone 5 and iPad (4th generation) Wi-Fi + Cellular
• iOS 10.3.3 —> Final release supported on iPhone 5C and iPad (4th generation) Wi-Fi
• iOS 9.3.6 —> Final release supported on iPhone 4S, iPad 2 Wi-Fi + Cellular, iPad (3rd generation) Wi-Fi + Cellular, and iPad Mini (1st generation) Wi-Fi + Cellular
• iOS 9.3.5 —> Final release supported on iPod Touch (5th generation), iPad 2 Wi-Fi, iPad (3rd generation) Wi-Fi, and iPad Mini (1st generation) Wi-Fi
• iOS 7.1.2 —> Final release supported on iPhone 4.
• iOS 6.1.5 —> Final release supported on iPhone 3GS, and iPod Touch (4th generation)