Applies to: Starter, Pro, Plus, & Enterprise Plans
Even though you might want to manually change the Completed On / Mark As data for one of your Learner's, don't forget that you cannot 'un-trigger' any potential automatic LMS consequences that may have already taken place, as a result of your Learner having finished their Course.
Owing to how Absorb works, making manual changes after the fact won't take back a Learner's awarded Certificate and/or revoke the automatic Enrollment in a follow-up Course, etc.
If you do really want to undo what's already been done - then you will have no choice but to go to the triggered end result (i.e. the Certificates Report, the Enrollments Report, etc.) and back out the outcome(s) for your Learner there personally.