Update ios 15.5 on my iphone

Update your iPhone, iPad, or iPod touch

• If you have an iPhone 5S or 6/6plus, the highest you can update to is 12.5.5

• If you have an iPhone 6S through 13, you can update to 15.5

Go to Settings > General > Software Update.

Update your iPhone, iPad, or iPod touch using WiFi -

https://support.apple.com/en-us/HT204204

Update using your computer -

Update iOS on iPhone - Apple Support

These are the iOS options based on iPhone Model --> You need to Identify your iPhone model - Apple Support

• iOS 15.5 —> 6S / 6S Plus, SE (1st Gen and 2nd Gen), 7 / 7 Plus, 8 / 8 Plus, X, XR, XS /XS Max, 11,11 Pro /11 Pro Max, iPhone 12, 12 Mini, 12 Pro, 12 Pro max, iPhone 13, 13 Mini, 13 Pro, 13 Pro Max
• iOS 12.5.5 —> Final release supported on iPhone 5S, iPhone 6/6 Plus, 

Learn how to--> Update your iPhone, iPad, or iPod touch - Apple Support