Downgrade from iOS 11 (beta) to iOS 10.3.2 and restore iOS 11 backup I've upgraded my iPhone to iOS 11, but I regretted from doing this. I've a backup saved at iCloud from Yesterday, and I would like to know if I reset my phone back to iOS 10.3 it's ok to restore this backup, even if it was made at iOS 11. There is something that I have to take careful or something that I will lose during the process?

Im okay about having to download all the apps again, I just dont want to lose the phone data (WhatsApp chats, Contacts, Photos).
You can't restore a backup from a newer iOS version to an older iOS version.

You should have an iTunes backup from before you updated iOS. If not, you will have to manually extract the data from your iOS 11 backup and add it back to your device after setting up your iPhone as new with iOS 10.3. How you do this depends on the individual app — many use iCloud such as iCloud backup in WhatsApp which will help you do this automatically, but otherwise it will be a case of manually editing a fresh backup of your iOS 10.3 device with an app such as iBackupBot. For things which are incompatible with older iOS such as your messages history, this will be unable to be restored to the older iOS.

Unfortunately all I can say is that you are warned of the perils of installing beta iOS prior to acceptance of the beta program. My suggestion is to put up with iOS 11 and keep updating to the latest beta as it becomes available, which will fix many of the bugs. You'll be updating to iOS 11 anyway in the future.