I downloaded iOS 15.6.1 to my iPhone X and still red security update notification

you say that you downloaded the update.. but did you install it?

What does it say if you go to Settings > General > Software Update?

Is it saying your WiFi is insecure?

If so, that's because you router is using a less secure security protocol.

Yes, at least 50%, better yet, leave the charger plugged in while updating.