A good easy way to get stains out of baking pans is to use baking soda and hydrogen peroxide.
Mix some baking soda and hydrogen peroxide together to make a paste and put the mix into the baking pan.
Then spread the baking soda and hydrogen peroxide paste mixture across the baking pan and allow it to sit for around 3 hours.
The baking soda and hydrogen peroxide paste will help loosen the stain and allow it to come off easily.
Then you can take a paper towel or washcloth or even a sponge and wipe the baking soda and hydrogen peroxide paste away.
The stain should come off the baking pan with it.
If not then you may need to scrub with some baking soda and vinegar or use some Easy Off oven cleaner but be sure to wash the baking pan off good afterwards.