This should the first side mission you gain for when you get outside onto the streets of Detroit. After you've spoken to David Sarif in his office, you'll need to head to your office where upon you'll meet Tim Carella. Quick note; Check your computer for a received email from Greg Thorpe.
This is a two part side mission. The save is for the two drug dealers Tims ex colleague Tindall needs you take out. Save before you deal with them just in case they go hostile. You can simply walk up to them, so try a stun gun + quick takedown combo on them.
I just climbed onto a roof, pulled out my tranq rifle and put them both to sleep, if you have that Augmentation which allows you to take down two people at once that works better, but yes in short you can just knock them out.
You don't have to kill them. Knocking them out is enough. If you don't have the multiple takedown aug, you can tranq the one with his back to the storage space, and if you shoot him in the arms or legs, you have a couple seconds to sneak up on the other one for a takedown. This won't endanger the no-kills or no-alarms achievements.
you dont have to do tindall's quest, you can just stun him and loot the data. (make sure you are hidden if you do this because any near by punks or cops that see you will go hostile.)hope this helps :)