First: Get a Sharpie or other Permanent marker, or colored nail polish.

Before adjusting the tension from your usual setting,
make a “match up mark” both the Silver KNOB at the top of the serger
on the left side in line with the presser foot bar and the actual top of the serger  with a line or a dot on both - so you can get the setting back to where it was originally.

Turning to the Right = Tightens pressure
Turning to the Left = Loosens pressure
Only a slight turn of the knob can make a significant adjustment
This is why marking both the knob and the top of the serger is really important.

From page 45 of the Manual