How to update my iPhone 6

iOS 12.5.6 is as far as the iPhone 6 can go. Is that what is already installed on the product?

If yes, the iPhone 6 cannot handle any higher updates due to its hardware limitations. You have reached the end of the update road for the iPhone 6.

Are you currently running iOS 12.5.6? Settings > General > About > Version. If you are that is as high as you can go, the iPhone 6 does not have sufficient RAM to run iOS 13 or later and you cannot add it.