How to change default Gmail account in android

(rooted device only)
open Root Explorer or Android terminal
go to root folder /data/system/
delete accounts.db


(unrooted device only)
do Factory Reset open Settings > Privacy > Factory data reset.
by this way, all our data will be lost including contacts, sms, applications, etc.

so think twice, what is your cell phone 'root' or 'unroot'