I can’t update my iPhone 6 there’s no update option

Go to Settings/General/Software Update. If you have an iPhone 6, your almost 8 year old phone cannot be updated higher than 12.5.5.

The iPhone 6 can only be updated to IOS 12.x.x series.

iPhone models compatible with iOS 15.1 - Apple Support

An iPhone 6 cannot be upgraded beyond 12.5.5, it doesn't have the hardware needed to run a newer iOS.