I have been working on a bathroom remodel in my 86, a few years newer but could be the same. I have a sink on a counter top. Is that what you have? Or is int part of a large plastic formed counter top?
In the sink on a counter top case, there were four screws that tighten a metal clamp. There is also a nut that clamps the drain pipe. After that the sink will pull up, hopefully there is some bedding compound holding it; not super tight but firmly. You can just push up and it should slowly release.