Hi
STEP*1:*Download and*install*iTunes 10.5 Beta 7*on your Windows or Mac computer.
STEP*2: Download the iOS 4.3.5 IPSW.
STEP*3: Now Put your iPhone in DFU mode and you’ll get the Recovery mode message.
STEP 4: Press Shift+Restore (Windows) or Option+Restore (Mac) in*iTunes and point iTunes to iOS 4.3.5.
STEP 5: Apple will automatically install iOS 4.3.5 on your iOS device without need to change anything in host file as it’s the latest public firmware.