I am not updated my phone 6s plus

One thing that you should know and confirm and that is whether you have an iPhone 6s or do you have an iPhone 6 (Without the S) This is important because an iPhone 6 can only be updated up to iOS 12.5.4 and NOTHING HIGHER. You need an iPhone 6s to update higher than that to iOS 14.x. Double check which iPhone you have.

Open Settings ➔ General ➔ About ➔ Model Name: What does it state there?

Axel F.