I did that before I tried to update Joomla to v3.3.5.
I opened CMS Update, turned off the backup (as one was done last night), and clicked on the button to update. The extension proceeded to show the download, then the extracted meter (number of files, bytes, and so on that have been extracted).
Then a red message shows up: Invalid Log In
Everything stopped at that point.
I tried again, same issue. I figured let me try the standard Joomla update, as I type this, its doing a backup using Akeeba.
On another site, I attempted the update using CMS Update v1.01, and it worked fine. Not sure if anyone else is having this issue, but I figure it should be reported just in case.