Hi snods, you should be able to Google instructions for opening up your particular Mac. (don't Google PRAM specific, just opening). Once it's open you can easily find the PRAM battery. It looks like a mini. "C" cell battery. It is easy to remove.
It is a coin battery, 3 V Lithium (922-6476), and requires opening the case to get to it. I cannot find a teardown photo sequence that shows how to replace it. Nor do I find the battery pointed out in any of the tears downs available at ifixit.com.