in one of my domains i have many email accounts(600) with quoata 5mbyte each of them...can all go automatic to 20mbyte because its very difficult to change them one by one.i am talking about cpanel control panel/WHM. panel as root..

SSH your server as root or that domains user.

WARNING!!
If you are not experienced with moving around in linux, find someone to do it. It can be dangerous, do it with your own risk.

Goto (change the bold areas with your info):
/home/account/etc/domain.com/
pico quota
you will see a file, contents like this:
account1:5242880
account2:5242880
account3:5242880
account4:5242880

change all 5242880 to 20971520
save.

I'd suggest almost the same method, but rather than editting each file manually, try

perl -p -i -e "s/5242880/20971520/g" /home/{user}/etc/{domain}/quota

to edit them all at once :)