OK
Above the triangles you'll see a graph - this shows the levels of your image - The left represents DARK colours, the right LIGHT
Ideally the graph should stretch from the left triangle to the right (indicating a good distribution of levels) - however, an old picture (like yours) which has faded may tail out out before either end.
Simply slide the triangles towards the point where the graph tails off to Zero (or thereabouts) to redistribute the levels - do this for LIGHT & DARK.
As you do this your image will change .
At the top of the box it shows the channel (default is RGB).
If you want to change the levels of the channels individually just select each one and move the triangles accordingly !
Hope that helps - it really is easier than I seem to have made it sound !