Cryptic is rewarding you for testing the Beta of the new ARC Client with a free uniform worn by Khan.
If you have already tested the Beta, then you already got it. If you have not tested the Beta of the ARC Client, you can simply download it.
The new ARC automatically detects installed STO versions, so you do not need to download it again or copy/move files on your hard drive.
The Khan uniform can be worn by all Characters, regardless of their faction. As proof, here is a Screenshot of Legion Admiral P'Triq Marqus wearing it.