Atticus produces print-ready documents according to the requirements of most major distributors like IngramSpark and KDP. We don't currently offer the ability to adjust top and bottom margins, but these are already pre-set to industry-standard sizes around .5in.
The inner and outer margins are controlled under Advanced Options in your theme settings on the Formatting Tab.
We've just updated Atticus to make this as easy as possible. Here's a helpful tutorial to walk you through the options available in your theme: https://www.atticus.io/create-a-custom-theme/