How to Disassociate an Apple ID from a work computer
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.