Most leather saddles have a tension-adjusting nut located under the nose of the saddle. Fortunately, this nut usually requires a special wrench, so most people leave it alone. In almost every case that I know of where someone has tried to adjust the tension with this nut, the saddle has been ruined. My advice is to leave it alone.
If a leather saddle gradually becomes too soft and too wide after many thousands of miles, it is sometimes useful to punch a few holes in the bottoms of the side flaps and lace them together under the saddle frame.
This allows the width and firmness of the saddle to be adjusted to the rider's taste. Some older models came with a row of holes along the lower edge of the side flaps, for this very purpose.
I realize that this sounds like a lot of trouble, but most cyclists who take the trouble find it well worth while--in the end.