As crich70 says, if you replace a file with a new version, the download count will be reset to 0. If you want to edit the post with the previous download count, make sure to note the number before uploading the new file, or you won't be able to see it again.
If you add a new file to the original post, the original file and its download count are not affected (as far as I know). When you "manage attachments" you will be able to add or remove files. Just make sure to add the new file with a different name, or it will overwrite the existing one.