If you're mechanically inclined definitely do the DIY linked by BMR and flush out the whole system. Just be careful of the brittle reservoir after all these years. The tube might crack off.
Otherwise just siphon and refill a few times over a week should do. Use an old shampoo pump like in the DIY and clean it first (I never realized these old pumps are so useful

Kudos to Ken4 in the DIY).
You can use any GM Dexron III suitable ATFs. But I'd stay away from the thinner Dexron-VI in these older systems.
Walmart's SuperTech Mercon-V works fine and its spec is very tight across brands (like GM's Dexron-VI). Just don't pick up an old formulation of Mercon-V gathering dust somewhere because the old formulation isn't Dexron-III suitable. Always read the label for "Dexron III" suitability.
But as mentioned, if you have an Advance Auto nearby and use the discount codes, definitely get the
Mobil-1 ATF or
Castrol Import Multi-vehicle ATF. Castrol ATFs are now called TransMax and Walmart has it too after disappearing for a while. Pricewise you'll be close to Mercon-Vs.