How to clean both sides of the glass on my flatbed scanner?

I started scanning pictures today and I noticed these marks coming up on every single scan. I got some glass cleaner and sprayed a soft cloth that I use on my prescription eyeglasses and cleaned the glass that the pictures rest on, but these smudges keep coming up in my images. I realized that the smudges are on the bottom of the glass. How do I clean both sides of the glass of my scanner so that these smudges and marks won’t show up on my pictures? Thanks!