How to downgrade Firmware in My Passport Essential?

I bought a My Passport Essential few days ago, after installing WDSmartware, it suggests me to upgrade WDSmartware and also Firmware, now every time I want to restart my laptop( Dell studio 1555), it is stuck upon  Dell’s logo appears till disconnect My Passport Essential !!!

Beside, I need my laptop boot from external hard drive to install windows 7

How to downgrade Firmware in My Passport Essential ???

You can’t.