To change a user's status in a course manually, go to Courses and click to edit the course you want. Once you click on it, go to the users tab (1). Over there for each student will will notice that the status of the user is actually a link (2). Clicking on that link will display a dropdown menu where you can change the status of the users.
Once you change the status of the user click on the confirm button to save your changes. If you want to cancel, click on the cancel button.
If you would like to bulk change the status of many users and also set a score, issue or revoke the certificate etc, you do not have to do the above process for all users. You can select the users you want (3) and then click on the "With selected" (4) option.
A pop up window will appear then and over there you can perform your bulk actions and change the status of the users.
In case that you have made changes to your course, for example if you have added new content units to it and you want to synchronize the users' progress, you can click on the sync icon under the operations column (5).