The thickness is controlled by the Extrusion operation. To change it, you must "edit feature" by right-clicking on that operation icon in the time line.
Or, I sometimes use what I call a "controlling sketch." In this case, put a little construction line on one of the vertical origin planes, and put a point on the top of that line. Dimension the line length. Then, when it's time to make the Extrusion, instead of a hard value for the distance, you can select the "to object" option, and click on the point at the top of that line. Now the extrusion thickness will be tied to the location of that point point, which in turn is tied to the length dimension of that line. You you can just turn on "show dimensions" for that sketch also, and control the thickness directly via the dimension.
I call it a controlling sketch because I don't actually make any geometry directly from it, but instead use it to control some aspect of geometry.