You create a new Civimail at this link.

A/B testing allows you to send two test mailings to a random subset of your recipients. After collecting and comparing metrics, the more successful mailing will be sent to the remaining recipients. You can test

• different "Subject" lines 
• different "From" lines
• different emails

To undertake A/B testing visit this link and set up the test, specify what percent of the mailings go to A, B and Final, select your target Group, and then compose the emails with the variations.