How to remove glare from glasses in phone photos?

It's still better to retake the shot if you can - it's more honest and simpler. But if the frame really worked out well in terms of composition and facial expression, you can try to fix it. On a smartphone, there are a few options: in free apps like Snapseed or Adobe Lightroom Mobile you can try to darken the glare locally using the "Healing" or "Removal" tool, though honestly, the result won't be perfect. If the reflection is really bright and large, you'll probably end up with some artifacts.

If you have access to a computer, you can do it more neatly in Photoshop or even free GIMP. There are tools like Content-Aware Fill that handle these tasks better. For small glares, a clone stamp will work - you can copy pixels from the neighboring glass with it.

But really - next time just retake it without flash if the lighting allows, or ask your friend to turn their head so the glare isn't pointing at the lens. It'll take two seconds, and the result will be way better than struggling with editing afterwards. After 25 years of working with people, I know it's easier to get it right the first time than to fix it later.

Retaking it is easier and the quality will be better, but if the shot is really good, you can try to fix it. I use Snapseed or Lightroom - they have a Clone or Healing tool where you can paint over the glare by copying the neighboring pixels. Though if the glare is big and really white, the result might look unnatural, because there will be some reflections on the glasses anyway. The main thing is not to overdo it and stop while it still looks normal.