First method: Position Lara to the left of the doorway near the boxes. From this spot, you can see the guard making roundes in the office, ending with his back to you. When he does this, activate the Stealth mode and walk inside the small hall, the get into the office. Sneak behind him and press Action to take him down.

Second method: With the entrance to your back, head forward in Stealth mode and use the switch to turn off the lights in the office. Still in Stealth mode, enter the room on the left of the switch and stand in the corner between the two doorways.

The guard will come out to see what is going on, so wait for him to pass and turn around the corner. He will stop near the switch, so sneak behind him and take him down.