iOS 12.5.5

There is nothing that can be done.

That phone is obsolete. It is physically incapable of running anything higher than iOS 12.

That model has now been discontinued for over 5 years.

iPhone 4S has a 32bit chip with 512MB RAM.

iOS 13 and higher requires 2GB RAM or more and a 64bit platform. iPhone 6/6+/5s has 1GB RAM

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

• iOS 15.2 —> 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