I am quite new at doing things with arduino so sorry if my question is stupid.
I want to use a led fading code that allow to adjust different speed for the fade in and the fade out. How could I proceed? Thanks for your help.
Look at the fade example that comes with the Arduino IDE.
If you need to fade several LEDs or do something else while the fading is happening look at the way to use millis() for timing in the Blink Without Delay example sketch and the extended example in several things at a time.