How can I re-qualify on a training mission?

From AAManual

It is possible to re-qualify on any training already done to attempt to improve on your previous score. After logging in with your soldier name, click the training tab on the left side of the main menu.

For basic training click once on the checked box that you would like to re-do. A new selection box will appear in the lower right corner showing retry training. Click on retry and proceed with the training session as done originally. After finishing the session, you will be given the option to save your updated score by speaking to the LT. If your new score is lower than previous attempts, it will not be lowered in your achievements record. You can try as many times as you would like to retrain.

For Infantry MOS Advanced Individual Training (AIT) click training as above and then click on Infantry MOS (Military Occupational Specialty). Next click on the training session that you would like to re-do as above, and click retry training in the lower right corner. Your new score will be recorded after completing the training.