To cancel “Unfriend”:
Given that you have not deleted the chat with the friend you have Unfriend, go to the chat > Friend’s name on title bar, then tap the more button on top right hand corner. Tap “Add Friend”. If your friend has not unfriend or block you, he/ she will be added as friend automatically.

To “Unblock”:
Given that you have not deleted the chat with the friend you have Block, go to the chat > Friend’s name on title bar, then tap the more button on top right hand corner. Tap “Unblock” > Tap “Add Friend”. If your friend has not unfriend or block you, he/ she will be added as friend automatically.