Hello, I’ve gone through the lighthouse lesson, but am trying to edit the length of the middle part of the lighthouse.
I’ve tried grabbing all the faces / edges of that section, then used the expand / grab function to revise the length (along the z axis), but things aren’t working 
How can I do this?? Thank you.
(Just in case I posted this question in the wrong place, I’m referring to Mr. Grant Abbitt’s “Complete Blender Creator 3” course, lesson “Making a Lighthouse”.)
It does not work like that.
If you want the middle section shorter select all the top parts and drag them downwards.
You can’t shrink or lengthen the length in the middle of two fixed ends.
Welcome to the community. Take some time to look around and share your progress! 
There’s a couple of ways to do this. The easiest would be to go into wireframe mode, select all the vertices at the top of the lighthouse, then move them up on the z axis.
The way you tried is kindof possible, but we would need to use a feature called “proportional editing”, which probably hasn’t been introduced on the course yet. If you wanted to do it this way, you could turn on proportional editing (the O key), increase the size of the circle using your scroll wheel, and then scale. This will scale the other parts of your lighthouse proportionally. It won’t have quite the same effect as the method above, but it won’t lead to the middle section expanding into other parts.
Good luck!
