You can not re-download transactions you have deleted.
If you think about it for a bit, you will realize that being able to do so would be a horrible mistake.
Learn how to deal correctly with downloaded transactions; or accept that you will have to manually re-enter them, if you make the mistake of deleting them when you should not have done so.
Quicken can not prevent you from experiencing the results of your own mistakes.
[I did not address the possibility that you kept the downloaded transactions in a backup of your Quicken data (or that Quicken, conveniently, made such a backup for you). Or kept a backup of your data from just before the download whose transactions you deleted. If so, all you need do, is Restore the appropriate backup.]