Since you obtained your battle tower, there definitely a skill trainer there in your tower, either Kenneth or Hermosa. To access your skill arena, use the dragon stone in your inventory, or press the U button on your keyboard, you will notice a small list pop out on your right screen. Select training arena and talk to your trainer. The 1st option in the skill training list is the skill reset option, which cost 5k gold for the first purchase and will double up for the 2nd purchases and onwards.
