It would help you to learn the basics of the codes in the .css file. It's not that hard, and then you would be able to fix a lot of things.
Find the class used for the section you want to change the line-height. For example, if you want the line-height for the paragraphs in a certain section to be 1.2, then look for the class name:
<p class="para">blah blah</p>
In this example, the class name is "para." In the Calibre Editor, click on the .css file at the left (probably named stylesheet.css) and look for .para. In that section, add the following line:
line-height: 1.2;
(or whatever line-height number you prefer...)
Continue to do this for all the classes in the sections you want to change.