I am not looking to move the demilterized land, or make a new one, but remove it all together, is there a way to do that? and can you explain it in a simple way. I am not good with modding.

This currently isn't available on the site. However, you can do this by removing the following line from the exported state file (if you have used it in a custom country):

set_demilitarized_zone = yes

If you haven't used the state in a custom country, you will need to copy the state file from PATH_TO_HOI4/history/states to your mod's folder, in the same history/states folder (you may need to create these yourself). And then remove the line above from the file you copied over.