How to do transmissions fluid change on 1982 rx7
 

Welcome aboard. There's nothing rotary specific about the transmission. There's a fill plug on the left side and drain plug at the bottom.
Take off the fill plug first because if it's really stuck you don't want to drain the oil and be in a bind. The head of the plug looks like a cube, use a 14mm open end wrench.
The drain plug is the large hex cap, I forgot the size, definitely bigger than 19mm.

I found it easiest to fill it by sticking a hose from the fill hole out through the top of the engine bay. Then you can stick the nozzle of the bottle into the hose or put a funnel on the hose and pour the oil down.

Also, download a service manual at foxed.ca

I agree with this 100%. I've never personally had an issue since I always do this from being trained in as a lube tech at Valvoline at 18 years old. Always pull fill plug first, that way you can fill whatever you're trying to drain. Much harder to fill something from a drain plug!

Follow j_tso's advice to the letter - and make sure to take the fill plug comment as Gospel.