The information below discusses the Sender/From testing variable. The rest of the details are covered in the A/B Testing Campaign Details article.
NOTE: tokens cannot be used in the overwrite From Name box.
You will need to set the values for both A and B.
Since this is the testing variable, please make sure they are different.
You can change the Sender/From until the message is within 5 minutes of the scheduled date/time of send.
You may also be interested in the Best Practices for A/B Testing Campaigns.