From version 3.7.0
On the iPhone first go to the 'More tab' , on iPad the option is located in the side menu.
Then choose 'Redisplay helpful hints'.
For older versions, first go to Help (Located on the left on iPad, or on the More tab on iPhone)
You can then choose to redisplay helpful hints (The grey overlays that tell you what buttons do) , or redisplay the walkthrough
