A dialog box will open that gives you several options for adding prerequisites and requirements.
Prerequisites are optional, requirements are compulsory.
Prerequisites cannot be added to the first module in a course. Prerequisites can be used in conjunction with requirements. Prerequisites will not trigger badge awards, only requirements.
After correcting that problem, you can either revoke badges that were awarded mistakenly or clear out the badge assigned to the module and reassign it. Just make sure before doing this that you have fixed the problem with the completion rules so that incorrect badges are not re-awarded the next time Badgr checks progress when you view the progress screen.