Hey Ceng,
So your Receiver (FS-A8S) doesnt Support PWM.
It only supports: PPM (I would use that with Flysky) , IBUS and SBUS.
To change to PPM (Hardware):
Hook up your Receiver to your FC:
You will have 8 Cables coming out of your Flight Controller.
Cable Number one is, where your Status and Power LED’s are.
Connect your Cables like this:
Cable Number one on FC: Ground (Black Cable of your Receiver to other black cable)
Cable Number two on FC: Power (Red Cable of your Receiver to other red Cable)
Cable Number three on FC: Signal (Yellow Cable of your Receiver to White Cable on your FC)
Should look like this:
(Pink is White because you cant see white…)
In this Case the other Cables are useless…
You can connect the Cables of the Flight Controller and your Receiver by simply soldering them together.
You also wont need the white cable of your Receiver if you use PPM.
If you want to use SBUS or IBUS just take the white Cable of your Receiver instead of the yellow Cable.
Software:
I have no clue 
Im pretty sure my answer didnt answered all of your Questions so maybe check out this to learn a bit more about your FC:
The ULTIMATE Guide : 
A great Video Series:
(next Videos should be in Autoplay)
What is PPM, SBUS, IBUS, PWM etc. :
same but tells a bit more about the wiring:
I hope i could help you a little bit
Luca