I want to update my iPhone 6 iOS to 13

Sorry, but that is not possible.

The iPhone 6 can not be updated beyond iOS 12.5.5.

Any iPhone that can't run iOS 15 can't run higher than iOS 12.5.5

iOS 15 compatible devices --> https://support.apple.com/HT212712

wait for future or if you find early way for updater please share with us too

thank you