Just replace my car battery and now the key fob is not working. But I'm able to start the car when you hold key fob next to the start button. Does anybody know the easiest way to fix this issue?

Try the following procedure, if it doesn't work it will probably have to be reprogrammed by a Nissan Dealer, Locksmith or a batteriesplus store.

Step 1: Lock All Doors Using the Driver's Side Power Lock. ...
Step 2: Consistently Insert & Remove the Key into the Ignition Until Your Hazards Flash. ...
Step 3: Turn the Key to ACC Mode. ...
Step 4: Push the Fob's “Lock” Button Once. ...
Step 5: Program Remaining Remotes. ...
Step 6: Turn the Key to Off & Remove It.

I will try that and keep you posted @04pathse

had to take it to the dealership.

If my car has a push start, how do I insert key into ignition????