Each Android phone is tied to a Google account such that if you make changes to the Gmail, Calendar or Contact, it will immediately sync with the main server. When you first set up the Android phone for usage, it will prompt you for the Google username and password and it will store it in its memory. However, there are times when you need to log out of your current Google account and log in to another one, you will find that there is no option for you to do that.
There are plenty of reasons that you need to log out of Google account:
1. Your current Gmail account has been spammed and you are switching to another account
2. Your Android Gmail is not syncing with the main server and you need to clear the internal cookie to get it resync again.
Here’s how you can log out of your Google account in Android and re-login again. (The following instruction is meant for Android 1.6. I will update this again when I have the chance to play with Android 2.1)
Go to Settings -> Applications -> Manage Applications
Scroll down until you see the Google App entry. Select it.
Press the Clear Data and Clear Cache buttons.
Exit back to the Home screen. You have just clear your Google cache and log out of your account.
To log back in, go to Settings -> Data Synchronization.
It will prompt you to enter your Google login credential.