How to update my iPhone's software?

An iPhone 6 can't be updated past IOS 12.

See the article(s) below. 

IOS 15.1.1 Compatability

IOS 15 and iPadOS 15 Compatible Devices.  *

iOS 14 and iPadOS 14 Compatible Devices.  *

iOS 13 and iPadOS are compatible with these devices - Apple Support.  *

Amandejoseph Said:

"Yes: How to update my software on the iPhone"

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Updating to iOS13 and After:

This in not an option. Your pertaining iPhone model (iPhone 6) is incompatible with iOS13 and/or iOS14  and/or iOS15. This is because your iPhone model (iPhone 6)does not meet the hardware requirements for these iOS version(s).

Assuming a compatible device, see: Update your iPhone, iPad, or iPod touch - Apple Support

If the above Apple article doesn’t help, please post back with relevant information.