How do I change assessment module settings globally across all assessment modules?
You will have to go in to each individual Assessment module and update the settings. The only case where this is not true is if you have just duplicated the one assessment module in to other courses using the 'link to' function. In this case you would just need to change the master module.
We built out a course all around creating Assessments in Litmos, check it out here: How To: Create an Assessment