Unfortunately there is no “Stop” button.
I had this same issue you’re having with the “VLC media player” Update (see topic: Software Updater in never ending “Preparing” status)
Once you “close” that “status window” showing progress you cannot get back to that specific window showing progress details. You will see in a more limited way any progress to the right of the program listing in the “Software Updater” module.
If you have problems with this feature (seems to hang for example with “Preparing”) eventually the “Update” will generally “ERROR” out in time assuming the updater fails.
You can try the “Refresh” on the upper left top of program list in the “Software Updater” module if it appears to be taking way too long. Sometimes this will resolve a “hang” that doesn’t get back to the status outcome properly.
If the issue is persistent, you can try doing a Software Updater “Database Reset”.
For Avast2014 go to “Settings” > “Tools” > “Customize” Software Update > “Database Reset (troubleshooting)” and do a “reset”. Doing this should bring programs needing an update to the top of the list (if they weren’t already).
As I said, I had the same problem your having with the “VLC media player” Update. I did a “reset” of the database (see above). I observed that this changed the order of the list putting the problem software (“VLC media player”) at the top of the list that needed to be updated. I then tried another update and it took what seemed to be too long once again, eventually I did a “refresh” (see above) and the status then changed so that it was checked off with an “Up to date” status thus presumably the update was successful finally.
I would try a “Database Reset” for the Software Updater (see above) and make another attempt at updating or just update directly from the website: LINK (This is what you get if you click on “details” in the Avast Software Updater while an update is in progress for the “VLC media player”)
Good luck.
P.S. In the event you’re using Avast2015: Resetting the internal database of the Software Updater
I had to look this up because I use Avast2014 (which is different than 2015 regarding the “reset”) and WILL NOT go to a version that has a “shopping cart” tab (unbelievable) but AFTER 15+ YEARS will finally find Virus protection elsewhere if this doesn’t change and 2014 is no longer supported. First we get “advertizing” popups that cannot be disabled in the “free” version which is bad enough (and we already had advertizing windows as part of the interface), now we have a “shopping cart” tab as part of the application interface in version 2015, REALLY, just WOW. If Avast doesn’t wise up and realize that people do not need to be continually prompted to add module options all the time I’m thinking they will lose business as I know of AV programs that do just a good a job or better without all the bombardment of ads. What Avast ought to do if they feel a need to “advertize” their products to their users is after 10 years as a customer for example to allow these annoying adds to be “turned off”, after all a user with them this long is well aware of the products available to them. Now I feel better getting that off my chest regarding version 2015 
As I say each time I post: I’ll put in my 2-cents every time I get a chance (since Avast doesn’t seem to care or get the message) regarding the annoying popup ads considering that I’m a very very long time user that is frankly getting fed up and will eventually move on if things don’t change with Avast regarding this harassment crap. The powers that be might consider putting in place “learning” software that eventually “gets the message” if popup ads are ignored long enough (it’s not like users aren’t aware of the products available to them after all). It seems that perhaps “greed” is getting in the way and ruining a reasonably good product, now I’m finding that some modules will actually “auto renew” membership costs and it’s like “pulling hens teeth” to undo the “auto renew”, unbelievable!!! Things are going from bad to worse for Avast, they’d better grow a brain as far as how their customers are treated because there ARE other good FREE AV programs available without all of this user harassment to squeeze more dollars out of users. What’s next, email campaigns to sell products? (maybe I should have not mentioned that :-X)