My iphone 6 plus is not showing software update.How can I update it.

Your iPhone 6 Plus can only be updated to iOS version 12.5.5. If this is the version that is already on your iPhone 6 Plus, then that is the end of the road of updates for that model. You will need a newer model iPhone to install iOS version(s) 14/15.

Axel F.

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 15.1 —> 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

You are welcome.

Axel F.