I have NIS 20.4.0.40, running Win 7 32 bit. Download Manager Never Stops

Using Support-New Version Check determined Version 21 is available, and download Norton Download Manager, run it to get the new version.  Did that.  The download begins, but when it reaches 100% is simply starts over.  By the time I got to it, it had downloaded about 3 GIGABYTES out of my plan's 15/month, and was starting yet again.  There was never an installation file to run.  I killed it & stopped the download.  OK, what do I do to get the update to ver. 21?