How to easily remove or replace white canvas dots.

I agree with Hatstead. Volunteers in this forum hate the way this forum suggests to attach files to your post instead of using the solution to embbed your file in the post itself via the 11th icon on top of the post window. Most won't open attachments, that's a big risk of downloading malwares.

Your issue is due to a directional lighting of the canvas. A diffuse soft lighting would be better.

It appears that it's possible to use a blurred layer copy in darken mode, which would lessen the effect of the bright dots.

To be more precise, I would create a mask to only show the bright dots. I like to use a white to black gradient map for that purpose, but a levels one would work.

Instead of a blurred layer copy, a layer with the filter Pixelate > mosaic in darken mode.

If you are so inclined, it would help if we could see a representative picture in its totality. If there are privacy concerns, you can blur the areas that you wish to protect.

I am thinking of replacing the entire background, but need to see the foreground as well.

When posting here on the forum, the best way is to use the "insert photos" icon at the top of the reply box.