If you are using the default password, you will see the message:
Warning: You are running FreePBX and Asterisk with the default manager pass. You should consider changing this to something else.
Running with the default password is a bad idea, simply because everyone else in the world knows it, and (if not properly firewalled, etc etc) could potentially connect to your asterisk box and do bad things(tm).
Warning: Don't get cute and try to use a password with non-alphabetic or non-numeric characters - things may break in strange ways if you try to use punctuation characters in passwords. Unless you really know what you are doing, stick to numbers and standard alphabetic characters. Also, you should probably read the comments below, to understand the importance of making a full backup before changing anything in case something goes wrong.
Changing the password
To do this, you need to edit two files: /etc/asterisk/manager.conf and /etc/amportal.conf
This controls the asterisk 'manager' users that are allowed to connect to the asterisk manager interface.
For full information on the file, see http://www.voip-info.org/wiki/index.php?page=Asterisk+config+manager.conf
You can have as many users in here as you'd like (for example, an operator panel might use one) and in fact, you should have different users for each application.
FreePBX requires a user that has a definition like the following:
[admin] secret = secret123password deny=0.0.0.0/0.0.0.0 permit=127.0.0.1/255.255.255.0 read = system,call,log,verbose,command,agent,user write = system,call,log,verbose,command,agent,user
There needs to be a corresponding entry in /etc/amportal.conf
Obviously you just need to use the same username (inside the square brackets) and password as above.
Once you have made the changes, you need to click on "Apply Configuration Changes" in order for the change to propagate throughout the system (If you don't see the orange "Apply Configuration Changes" bar, go to one of the GUI screens in the system and re-submit the page, no changes necessary). If you don't do this, then extensions_additional.conf will have stale data resulting in a broken phone system.