The steps you should follow are:
- First you have to allow the test repetitions through the test options (1).
- Then you have to specify a maximum number of attempts or allow unlimited attempts (2).
In the first case the system will keep all of these attempts while in the second case it will store the 5 most recent.
- Visiting the Test reports and assuming that there are users that have taken the test, just click to preview the test for the user you want and then select the attempt (3):