I want to update my iPhone 6 ios13 now

You can't.

See the list of compatible devices:

iPhone XS Max

iPhone XS

iPhone XR

iPhone X

iPhone 8

iPhone 8 Plus

iPhone 7

iPhone 7 Plus

iPhone 6s

iPhone 6s Plus

iPhone SE

iPod touch (7th generation)

Sorry, you cannot. Your phone is too old and no longer supported.

If you want iOS 13, you'll need a newer iPhone model.