Summarize the following:
The WhatsApp icon looks like a green box with a white speech balloon and a telephone icon in it. This button looks like a gear icon in the lower-right corner of your screen. It will take you to your WhatsApp Settings menu. This option has a blue key icon next to it. Tapping on it will open up your account settings. This option will show you the number of contacts you have blocked, and tapping on it will open up a list of all blocked contacts. You will see the option to Unblock to the right of your contact's name. Alternatively, you can tap on the blocked contact's name on this list, and bring up the Contact Info page for this person. You can see some of your conversation details like Starred Messages and Groups in Common. There will also be an option to Unblock this Contact at the bottom of this page. This is the red button that appears to the right of your contact's name when you swipe left on it. Tapping on Unblock will remove the block. This person will now be able to call you and send you messages on WhatsApp.

Summary:
Open WhatsApp Messenger. Tap the Settings button. Tap Account. Tap Privacy. Tap Blocked. Swipe left on a blocked contact on the list. Tap Unblock.