When will my phone be updated to iOS 15.7 ?

Maximum iOS for iPhone 6 is iOS 12.5.5 (16H62)

An iPhone 6s can get the latest iOS.

BDAqua wrote:

Maximum iOS for iPhone 6 is iOS 12.5.5 (16H62)

iOS 12.5.6

An iPhone 6s can get the latest iOS.

iOS 15.7. The latest is iOS 16.0.2

To Follow on.

To get the Newest version of iOS or a newer version of iOS will require a New or Newer iPhone.

You could use the Apple Trade In Process for Your Region to purchase a New iPhone and use the Trade In value of your existing iPhone 6 against the cost of the new iPhone.

Note: Have used this process for both Apple Computer and iPhones over the years and it has worked very well for my needs

iOS 13 and iPadOS are compatible with these devices - Apple Support (CA)

Sorry but this iPhone does not qualify for iOS 13 and would there for not Qualify for iOS 15.7 either.

Thanks. :)

You're welcome.

I believe BDAqua mentioned that.