How to update iPhone 6s Plus to iOS 12.5.7?

If your phone is currently at iOS 12.5.7, then you don't have an iPhone 6S plus, you have a 6 plus which cannot be updated any further.

Go to Settings > General > About and see what model you have.

We are confused.

Your profile tells us that you have an iPhone 6s Plus. You are asking about an iPhone 6 Plus.

Do you have an iPhone 6s Plus....or....do you have an iPhone 6 Plus?

Your phone cannot install any higher iOS than 12.5.7.

If you want to run higher iOS versions, you will need a new phone.