Go to options, controls and then set the attack key to what you want to use to fire the bow. You can also use hotkeys on arrows to fire them even if the bow is not equipped.
On keyboard: use arrow keys to select the skill you want to allocate points to and hit left/right to allocate. With mouse: click the level you want. Note that you need enough skill points to select the level, and some skills (like attack) are expensive.
 "Goldsmith"){kind=icon}
 "Aground"){kind=icon}
 "Villager Kings"){kind=icon}
 "Elephant Quest"){kind=icon}
 "The Illusory Wall"){kind=icon}
 "Swords & Potions 2"){kind=icon}
Go to options, controls and then set the attack key to what you want to use to fire the bow. You can also use hotkeys on arrows to fire them even if the bow is not equipped.