To remove or replace a file in the build list you must first check to see if the pack is published.
If it is, it will need to be unpublished before its content can be changed.
With the pack unpublished, go to the 'Build' step to review your source files. With your mouse, click/grab and drag the file in question to the left and over the source file panel until it becomes active (it will turn red).
Release the file and it will be removed from the sectioned build list and re-added to your 'source files' list.
To delete the file, click on the three black vertical leader dots to the right of the file name and choose Delete.
Tip: If the name of a source file is greyed out in the source file panel, it's being used in the build step, or the meeting is in a status of published and cannot be deleted. If the file name is black, it is not in use and can be removed.