Moving this discussion to the Acrobat Installation & Update Issues forum.
Please refer to following links, might be helpful.
http://answers.microsoft.com/en-us/windows/forum/windows_vista-windows_programs/acrobat-co m-will-not-update-getting-error-16820/7bbbef64-df3e-4fbd-b552-fc1c7a5983c3?msgId=dcf838d8- a198-4a3a-b034-94a07c2e8648
If this doesn't work, we need more information here.
As mentioned, a lot of information is missing. What version of Acrobat (include subversion)? What version are you trying to update to? Are you trying to skip updates? (generally you can't skip) Are you doing the update from the help menu or by downloading the updates. Often the latter ends up being necessary.
Sounds like you have the Acrobat.com desktop (AIR) application installed. This application is no longer supported. I would suggest you uninstall it and simply use the web interface (http://acrobat.com) moving forward.
Sorry for the inconvenience.