If you manually change the Completed On / Mark As data for one of your Learner's, you cannot 'un-trigger' any automatic LMS consequences that may have resulted from your Learner finishing their Course.
For example, making manual changes after the learner completed the course won't cancel a Learner's awarded Certificate and/or revoke the automatic Enrollment in a follow-up Course, etc.
If you want to undo actions that have been triggered, you will have to go to the triggered result (i.e. the Certificates Report, the Enrollments Report, etc.) and manually change (delete) these outcome(s) for your Learner.