How do I disassociate an Apple-ID from an iMac? I am trying to push updates on a work computer but I can not. I am unable to because a previous employee's apple ID pops up and I do not have the password. Does anyone have any suggestions?
Simply put, you can't without the Apple ID info of the original owner/user.
However, there is a workaround. If you reinstall the original OSX associated with the machine using Internet Recovery, this will enable you to sign in using your Apple ID and subsequently upgrade the OS from there. Note that this will not work using the simple recovery mode (the partition already on the disk).
Although reinstallation is non-destructive, it's always a good idea to back up beforehand.
There are a number of ways a Mac might be linked to a particular Apple ID, and how you disassociate it depends on how ir's linked. Installing system updates shouldn't require any Apple ID. Are you updating apps that were purchased from the App Store? If so, those apps are permanently linked to the Apple ID that purchased/downloaded them, and the only way to remove the association is to delete the app(s) and re-purchase/re-download them under your Apple ID.