In order to retake GMAT Prep tests, you have to reset them.
On the screen in GMAT Prep that shows the tests you have taken, the Take A Practice Exam screen, in the lower left corner, there is a Reset button.
On another note, be clear the GMAT is not a test of concepts of knowledge, but rather a test of skill in figuring out how to get to answers and making decisions. So when you retake tests, to the degree that you see repeated questions, even ones the answers to which you don't fully remember, you are not really being tested in what the GMAT tests. So while repeating GMAT Prep tests once can be a good idea, repeating them more than once can actually be rather counterproductive.
Here's a blog post on resetting and repeating and the results of doing so that I have seen.
https://infinitemindprep.com/resetting-a ... et-smoked/