There were some specks of dust that appeared on the optical viewfinder when I looked through it. I tried using a blower but it didn't work and I may have introduced a scratch to it. Is there a way that I can replace the viewfinder's glass?
You probably can, and a Canon service centre would be able to do it, but it would be worth weighing up the cost, if it's in the optical viewfinder, then it won't affect any images because the mirror that is behind the lens that passes the image to it gets raised when the image is captured to send the light coming in through the lens onto the image sensor instead of the viewfinder
This is not a user-replaceable part you can easily purchase and replace. You can try to do it with an aftermarket 3rd-party screen, but be sure you know what you're doing--these things scratch very very easily. The XXXD dRebels aren't like the pre-70D XXD bodies where you just swing a latched door open and pop the new focus screen in. The focus screen of my XT/350D was held in place by a very fragile brass spring/frame that it took a relatively delicate touch to remove without scratching the focus screen. I got it out ok, but ended up scratching the plastic--very easy to do, given the matte textured surface it has--by trying to clean it with a microfiber cloth.
I just shot with the scratch on the focus screen, since it didn't affect the images, for about three years, until the camera's power board failed, and I had to send it in to Canon Service anyway. At that time, the flat repair rate then covered both the power board replacement and a new focus screen.