Please how do I update my iPhone 6

Most likely, it's because it requires a newer version of iOS. As stated, your iPhone 6 cannot be upgraded further. It is hardware limited and I would seriously doubt that Apple will provide any means to change that. You will need a newer iPhone model. Currently, you would need an iPhone 6s or later to run the current version of iOS.

You cannot update an iPhone 6 beyond iOS 12.5.3. If you need to run apps, requiring a newer version of iOS, you will need to get a newer iPhone model.

Your iphone 6 can only be updated to ios 12.5.3