I want to update my phone to iOS 15.0.2

That's not going to happen.

An iPhone 6/6plus can't be updated beyond 12.5.6, it doesn't have the hardware needed to run anything higher.

If you need a higher iOS version, you will have to purchase a newer phone.

You cannot update to iOS 15.0.2.

You can update to iOS 15.7

Settings > General > Software Update

The iPhone 6 cannot be updated the way you ask. It is too old.

Please read the reply above.