How do I change my paragraphs from skipped lines to indented paragraphs, or vise versa?

You can control the paragraph separation setting for your book for each of the three main book formats you can create in Pressbooks:

• Webbook
• Print
• Ebook

To change this setting: 

1. Hover over Appearance on the left sidebar menu of your book's dashboard
2. Click on Theme Options
3. Choose the format tab at the top of the page (ex: PDF Options) 
4. Choose your desired paragraph separation setting next to Paragraph Separation
5. Click Save Changes
6. Re-export the file