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First, you must back up your HTC Desire Z information.
Google has your back. Go to Settings > Privacy, and make sure that “Back up my settings” and “Automatic restore” are checked off. Go to Settings > Accounts and sync, open your Gmail account, and check off all options. With these settings in place, your contacts, system settings, apps, calendar, and e-mail will be restored whenever you set up a new Android phone with that same Gmail account.
Google hasn’t implemented a native photo backup service yet, so look to third-party apps to safeguard your photos. You might want to consolidate your mobile photos with those you already have stored in a cloud service. For instance, Photobucket Mobile will automatically upload newly snapped photos in the background to your Photobucket account. Flickr Companion and Picasa Tool are also free apps that allow mobile uploading but don’t do so automatically
Then, check your HTC Desire Z Current Software Version and update through Windows and Mac computers using the Kies software application.
If you have not already installed Kies on your computer, click http://www.samsung.com/us/kies/ to go to the Kies page.
When you download and isntall the Kies,then Start it and Connect HTC Desire Z to your PC.
Ensure that Kies recognizes your device by looking in the upper section of the left-bar navigation menu. Your device should be listed.
Once Kies recognizes the device you will see the update page.
Click Update and read through the Caution pop-up and, if you agree, click to populate the I have read all of the above information checkbox.
Read the Allow saving of information statement and select Allow saving or Proceed without saving and then click Start upgrade.
Kies will begin downloading firmware upgrade components from servers. This will take a few minutes depending on the speed of your Internet connection.
After a few minutes, the firmware upgrade will be completed.
Click OK. You can now disconnect your HTC Desire Z from the PC. Once the device has rebooted,HTC Desire Z will be ready to use.
Last,Kies can help you restore the HTC Desire Z OS,of course, you can also use Google to restore.