How to install this ball valve
The reason you don't have a union is that the previous installer assumed you would simply unsolder the elbow when you needed to replace the threaded valve. That's pretty much what they had to have done (well, really just solder the elbow after the threads were tight, the first time it was assembled.)

So, you can learn to solder and try to squeeze a union in there, or you can learn to solder and just disconnect the elbow, tighten the threads, and reconnect the elbow. 

Either way works.