How do I go about changing the ac condenser fan (the one in the outside unit mounted on concrete slab)? It does not seem to be spinning except once in a long while. Also, will this fan not turning cause damage to the other parts of the system? In other words, how urgently should this be done? Thanks.

It's the motor that needs changed, not the fan itself. It should be changed as soon as you can as it will make the rest of your system work harder and waste electricity trying to cool your home down as well. You could end up with another component wearing out in your system as the fan motor fails.