Select the "Weapons" option at the menu. A certain amount of teeth are required to upgrade weapons. Select the weapon to be upgraded and an the option to upgrade it will appear if you have enough teeth collected. It is recommended that the Vorpal Blade be upgraded to level 4 before any other weapon. It is your main weapon, and you will use it most frequently. All the other weapons work fine at their lower level for their specific purpose, such as hitting Eyepots, etc.