To prevent glare a pen display will have either:
An etched glass surface
Or a anti-glare film.
Besides reducing the glare, films have the added benefit that they prevent the glass underneath from being scratched. Also the films can be removed and replaced.
This document covers the case when your tablet has an anti-glare film.
If it has an anti-glare film, the film can be carefully lifted off.
You can start at a corner and gently and slowly start lifting it up. And then slowly working your way across the entire surface.
First, you should realize than manufacturers might sell replacement films for their tablets. But even if they do not, you can find third party films that are pre-cut for your tablet or for which you can order them cut to a specific size. More here on your options: protective sheets
How much anti-glare sparkle do you want?
Your replacement will will likely have some amount of anti-glare sparkle effect. If you buy the replacement film from the manufacturer you will get the same amount of ag sparkle as you originally had. More here on anti-glare sparkle.
If you want less ag sparkle, you should consider getting a film which has less. This is where you should evaluate options from companies like photodon. They sell little samples you can try out to evaluate how much the film will look on your tablet. I highly recommend trying those samples first before you commit to putting a new will film on.
Here's a video where you can see me try some of these photodon samples and talk about how it affected the ag sparkle: