Replaceing existing stadium (all stadium that have an EA original ID) is quite straightforward. You just need to rename the stadium files you downloaded to match the ID of the stadium you want to replace (don't forget to edit glares). You can use the script I shared on the the software section of this forum to do it in few clicks. Once the stadium files have the correct ID, all you need to do is to copy the files in your game folder (data/sceneassets/stadium and fx and crowdplacement), then you need to regenerate with the tool of your choice. I usually regenerate with CM16.