How can l change iOS 12.5.4 to iOS 14.5.1 on iPhone 6s?

It would seem by your profile that you have an iPhone 6 Plus. The iPhone 6 Plus cannot run an iOS higher than iOS 12.5.4. You cannot update an iPhone 6 Plus. If you had an iPhone 6s, it would allow you to update.

Be very careful.

You have an iPhone 6+, not an iPhone 6S.

These are very different. Yours cannot be updated beyond iOS 12.x.

You can't. That model isn't capable of 13 or above.

(200872)

You’re welcome. Enjoy your iPhone.

You're welcome.