Each skill has a hotkey attributed to it by default. What if you want to swap their places? To do so, open the skill panel (K) and drag the skill icons to the bottom panel to replace the skills already assigned there. This can make it easier for you to use skills with cumulative effects, similar cooldowns or exchange old skills with new ones. You can manage items, such as potions, during battle in a simillar way - by dragging their icons from the inventory panel (I) .