i reset my iphone without update i want to update

An iPhone 6 can be updated to iOS 12.5.5

Follow theses instructions to do so, using Wi-Fi or your computer.

Update your iPhone, iPad, or iPod touch - Apple Support

Are you talking about a model 6, still running iOS 10? Have you, by any chance, found this iPhone in mothballs?

Bear in mind that that iPhone model cannot supports iOS versions beyond 12.5.5, because of hardware (CPU/RAM) incompatibility.

See these instructions:--> Update your iPhone, iPad, or iPod touch – Apple Support.

Regards

Giulio