Once TR imported to to QA system, then you can't revert the TR.
You have two options.
1. If you want to restore the changes then you can re-transport the previous TR.
Suppose you have imported a TR (xxxxx) previously. Now you have made some changes and create and transport a TR (yyyyy) to Qa system today.
Now you can re-transport the TR (xxxxx) again to QA system, the new changes for TR (yyyyy) will be removed.
2. If you know the changes which has already transported to QA system , then you can make another new TR for the same (contain the reversing data) and transport it to QA system.