Air data rate nRF24L01 I'm using two nRF24L01 to communicate with each other. I did the right setup and they worked fine, however I wanted to change the air data rate to a lower value, for more stability, as the datasheet says.

How can I setup the configuration for this?

After a small research I knew that you can change this using RF_DR 
to change the value of the transmission, but I didn't really know how I could exactly write the correct code for this. 

Another question: does this effect the range of the module? Or does it only affect the stability? And if not, then which code can I use to extend the range of these two modules ?
Increasing stability will increase the (usable) range. At least a bit. You could try decreasing the packet size, to make it even more reliable. There is no way to increase the output-power (only decrease). 

You could try different channels, and see which one gives you the best range in your application. 

After that you have to get NRF24 modules, with external antennas and amplifiers.