Plum tree help

Hi there,
We bought a house recently with a huge garden.
Tucked away between some fences and a shed is a mildly productive purple plum tree.
Any ideas on how to make it healthier?
It has a handful of plums now and is hard to reach.  

Welcome to Permies!

Aren't you lucky to find a plum tree already waiting for you!  I expect that someone here knows the answer; it probably has something to do with pruning and giving it space.  I, however, am not that person.  I'm just here to say hi.

You might want to give a hint about where you are, or at least your USDA zone.  It helps people when they give out agricultural advice!

Glad to have you!