Ok I have this figured out, I used a jewlers screwdriver and slid behind the tab and moved the spring back while prying the top open to slip out the old coupler and then slid the new coupler in sliding out the screwdriver and allowing the top to close back up. 

Bach-Man,
I do not see any metal clip on the coupler.  It appears to be totally encased in plastic and while there is the t shape to the end I see no way of getting it out.  Thanks

I just purchased the Bachmann Crane & Boom and it came with both rapdio and knuckle couplers but I cannot figure out how to remove the knuckle and install the rapdio.  Any ideas??