Open up your menu and select the character you want to change. You'll see: Change Equipment, Change Wield Style, Check Arts, and Check Items. Select Change Wield Style. Make sure you have the correct weapon equipped because some weapons are Power Grip while some are Two-Hand. And with that, you can only use 1-handed, Dual Wield and Power Grip with a 1-handed weapon. To use 2-handed weapon styles requires a 2-handed weapon.