All you need to do is click on the Course that you want to manage. Navigate to the Users & Groups tab, then click on Bulk Actions.
From there, click on Download CSV Template.
Open the CSV file that was just downloaded. From there, you can edit the course enrollment and completion dates by editing them using Excel, or another similar program. You can also enroll users directly into the course by entering their email in column A and entering a 1 in column E.
Save your changes, then click on Choose File, select the file you just saved, then hit Upload.