Here's now to create a chat history backup on WhatsApp for Windows Phone.
1- Open WhatsApp and tap the three dots at the bottom-right
2- Select Settings >> Chat Settings >> Backup
(This will create a backup of your WhatsApp conversations).
3- If you accidentally deleted your chats, we suggest that you don't create a new backup. Instead, check the time of the previous backup, which can be found under the backup button mentioned in the previous step.
4- If this time is after you received the chats you deleted, uninstall and reinstall WhatsApp
5- After verifying your phone number, WhatsApp will ask you if you want to restore a chat backup. Select Yes.