Sorry bludiamond.
Since my phone is rooted, I did not want to install this update. It's quite upsetting that AT&T forces the update unconditionally.
Anyway, I had let the download to complete, but did not let it to install. The update did prompt me, whether I want it to install now, or later. I had opted for later.
Then, after some Googling, I had deleted the download (from /sdcard/Download), and had un-installed the following services: com.htc.demoflopackageinstaller and com.smithmicro.dm.
Then, I've rebooted the phone. Afterwards, the phone still had a notification, that there's an update ready to be installed. When, selecting the notification, updater did realize that the downloaded file is gone, and had prompted me whether or not I'd like to download the update again, on which, I had responded no. The notification is gone, and AT&T did not prompt me to download and install the update again, at least not yet.
My phone seems to be OK.