When will my phone be updated to iOS 15.7 ?

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

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

An iPhone 6s can get the latest iOS.

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.

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

Thanks. :)

You're welcome.

I believe BDAqua mentioned that.