reduce max rpm on 912S2

You will need to either Reduce the throttle or increase the pitch of the propeller.

The real question is, "What RPM are you actually seeing?"

Yellow does not mean much!

As long as you are below 5800 rpm at Wide Open Throttle, and below 5500 at cruise, there is no problem.

Thanks for your answer, but how can I reduce the pitch to a fixed pitch propeller or how can i reduse the throttle?  any help? 

You would Reduce the throttle by just throttling back.

You would INCREASE the pitch of the prop by replacing it.

AGAIN! ...   "What RPM are you actually seeing?"

What is Green?   What is Yellow?   What is Red?

If we don't know what YELLOW means, it is possible that you need to adjust the colors on the Tach rather than changing the RPM.

Are you going into the Yellow during Climb, Descent, or Cruise???

green ranges from about 600 rpm to 2250, yellow ranges from 2250 to 2380rpm, here is (2380rpm)  where the red line is crossed. I'm going in to the yellow during cruise. I did a test run and found that it needed pneumatic syhronization after that the rpm returned into the green area, now i'm reading just over 2200 rpm.

I will assume that you are quoting Prop Rpms.

It is far more common to quote Engine RPMs but OK!

The Yellow Zone could be 2260 (5490)

Going above 2260 (5490) at cruise, at Wide Open Throttle, is to be expected.

You should be in the Yellow, 2260-2380 (4590-5800) during the climb, so exceeding it after you level out is absolutely normal.

Your Yellow Range, 2260-2380 (5490-5800), is permitted for up to 5 minutes at a time. 

It is Not something to be avoided, just used with discretion; Usually during initial Climb.

If you are not able to make 2300 (5600) RPM at some point in the flight, you actually need to raise your RPMs, Not Decrease them.