Readynas 316 fan push or pull?

Normally rear-mounted case fans pull air out of the chassis, while any front or side fans pull air into the chassis (creating front->back airflow).  If you set up the main fan to push in cooler air, then the PSU fan immediately exhausts much of that cooler air - so it will never reach the electronics. 

You can easily check the orientation by simply putting your hand near the fan egress.  I recommend keeping the fan orientation that you have now.  

Changing the fan voids the hardware warranty, so I suggest postponing this change untill the warranty expires.  The fan should be covered by the hardware warranty, so if it is failing you could also request an RMA.

Silent Wings 2 PWM has lower fan speed and airflow than the cuurent one used on RN316.

It maybe have issue when ambient temperature raised.

So I went ahead and replaced it the other day, in pull configuration. I noted that the board at the back would pretty much block all air flow to the drives if it were to push air in.

I tried beforehand putting my hand to it to check, but the airflow was pretty minimal, and I've felt fans that had some air flowing out when they are pushing air in, so I wasn't quite sure when I did that.

I looked up the specs on the stock fan using the part number on it, and found that it flows in the low 20CFM area, the Silent Wings 2 PWM 120mm fan is 50CFM at 16.5 DBa, 1500RPM. I didn't pay attention to the speed of the stock fan, but with the air flow rate of both, that is irrelevant.