Use of left nozzle?

I’ve just installed the second extruder on my printer.
But now, I can’t find how to use it for a print… Where can I select which extruder to use, if only printing with one color? (In the manual I find only how the print with two extruders in one part, not how to select the left or right extruder only)
(I did change the firmware back to two extruders).

Many thanks!

You choose nozzle 1 or 2 in Repetier .
Object placement -> Load model then choose EXTR 1 OR EXTR 2
Slicer -> Print Configuration, choose SINGLE HEAD