Open a map in TuneECU.
Click on "Map Edit" at top right.
On left hand side, click on "Idle"
The top row is engine temperature in degrees Celsius.
The bottom row is engine speed at each temperature.
Click on the engine speed at the temperature that you want to change, and use up or down arrows to change. (I would only suggest altering the three on the right - at most.)
Save, then send to ECU.
I hope this helps,
-Wayne