How do I do this part of "Introduction To SolidWorks" tutorial?

1. After I sketch the first circle, 128mm, and then make it 7mm thick, how do I select that diameter, 128mm, and change it to some other value (say, 170mm)? It seems to me that I can’t do that once I have given it thickness.

2. What does the word “boss” mean in this context (within the solidworks world)?

3. After I have made a circle, and given it the 7mm of thickness, the object is just sitting there. If I click on “Smart Dimension” I see the following list in the property manager box:
a. DimXpert location dimension
b. DimXpert size dimension
c. DimXpert basic location dimension
d. DimXpert basic size dimension
e. Reference dimension
I assume this stuff is beyond what I need to know for now. The reason we keep seeing this is I find a dimension on a student’s work that is wrong, and I want to correct it. I can’t seem to be able to just use “smart dimension” to change a dimension unless they are just adding that thing. Once you give it a dimension and click the green check, then go on to something else, it seems to be set in stone.
But that definitely feels wrong to me.

4. When I am trying to “Extrude the ring boss”, students naturally click all over the drawing before they get to the next step. I do not know how to re-select just the ring and not the surface contained by either circle before going on to set the extrude depth to 12mm.