Go to Settings > Mail, Contacts, Calendars.
On the account screen, tap the Exchange account you want to remove
Scroll down and tap Delete Account. A Delete Account warning will open
On the Delete Account warning window, tap Delete Account to finish. You have successfully deleted your exchange account.
Android:
Use the instructions below to remove an exchange account on your Android device. These instructions apply to devices running android 2.2 or above. Note: Each Android version can be slightly different.
Swipe down from the top of the screen to pull down the menu.
Select the Settings icon that looks like a gear.
In the new menu, click on Accounts.
Select the account you would like to remove.
Select the Remove Account button.