If you are unsatisfied with your stat allocation, you are able to reset it in each area's tavern/inn. Example is Chief Merrick as shown at the image below:
Talking to him will give you the option to reset your stats as shown here:
Tapping on that will allow you to reset your stats but it will costs gems to do so.
NOTE: The initial cost for resetting stat points is 50 gems and the cost will increase with successive stat reallocation.
If you accidentally switched screens on your device and the stat allocation window is closed, just complete the first level in the first map. Once completed, the stat allocation UI will pop up and prompt you to allocate your stat points.