a. When an Instructor accepts the answer
b. When uploading an answer
When you set up that the assignment unit can be completed with option (a), you give the ability of being able to modify the answer to your learner, if the Instructor has not graded him yet (1).
Now all attempts from a single learner can be found in the Assignments Reports (2).
Just click on the assignment and then view the answer of the learner (3).
If the learner has more than one submission, you will be able to see all of their attempts but also the date and time of the modifications (4).