I have a apple iphone 6s 128gig but does not want to upgrade to latest iOS 15 it’s stuck on 12.5.5 please help me to get it to iOS 15I have a apple ihpone 6s 128gig but does not want to upgrade to latest iOS 15 it’s stuck on 12.5.5 please help me to

If you cannot update your phone past iOS 12.5.5 it is not a 6S, it is a 6 (or 6 Plus). iOS 12.5.5 is a special version of iOS 12 released exclusively for phones with 1GB RAM (iPhone 5/5S and iPhone 6/6 Plus). All subsequent versions require a device with 2GB (or more) RAM.

If you need to use apps which require a newer version of iOS, you will need to purchase a new phone. Most - if not all - developers have long since stopped supporting devices with 1GB RAM.

Your profile says iPhone 6 Plus, Your post says iPhone 6S Plus.

iOS 12.5.5 is not possible on iPhone 6S Plus.

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

• iOS 15.1.1 —> 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

It sounds like your model is a 6 Plus (not a 6s), which cannot be updated past 12.5.5 version.

See why in this compatibility list: