AdvancedSetup and mbressman, thanks for your responses and suggestions. It looks like the problem is fixed and the Avast virus protection can again update. Here is what happened:
Per AdvancedSetup’s suggestion, I temporarily disabled Avast’s real-time protection and then attempted to remove instup.dll from Malwarebytes’ quarantine. Was still unable to remove the file from quarantine.
I went to the link AdvanccdSetup provided and downloaded an Avast installer file using one of the links from the web page. I ran the installer file without first uninstalling Avast, which I was reluctant to do unless necessary. When I launched the Avast installer file, right away Malwarebytes quarantined it.
I then went to the Avast web site and downloaded an installer file directly from Avast. It didn’t look promising when I saw that the file name was exactly the same as the first one I downloaded that Malwarebytes quarantined, but this one ran. Shortly it displayed a message that it couldn’t work because the file instup.dll was missing, but it continued to run. When it finished, from all appearances the updated Avast software was present with updated virus definitions. The file instup.dll is back where it should be and from what I can see to this point, everything is working now.
mbressman, I think you are correct about the reason I was seeing multiple instances of the instup.dll file in the Malwarebytes quarantine. After attempting to remove it from quarantine I wasn’t seeing it back where it belonged and Avast continued to say it couldn’t update. I still wonder why I wasn’t able to remove the file from the Malwarebytes quarantine. I think you are also correct about not needing to be concerned about the file continuing to display in the quarantine list after the Avast install ran and put the file back in place, since Avast now seems to be working since that was done.
Again, thanks to you both for your help. AdvancedSetup, my compliments to you as well for the detailed instructions you gave on what to do and how to do it, including the links you provided to what you were suggesting for me.