How to modify the dimensions of a part that is already extruded.

You need to edit the sketch. There you can add your smart dimensions. From there you would only need to double click the feature to be able to change the dimensions.

There are 2 ways:
1. Double click the (extruded, revolved, etc.) part, then change the dimensions that'll appear.
2. Click on the arrow before the (extrusion, revolve, etc.) command (in the FeatureManager Design Tree), then right-click on the sketch and choose "Edit Sketch".

For more illustration, check this video:
https://drive.google.com/open?id=1bMaJqTKT8-g2mzB0jg1CgcQATKQKM85Q