When i apply overgrip for my racquet, do i need to peel the old one off????

ok, i have a Wilson racquet (aus open one) and it originally has a white grip and i find it slippery and i'm deciding to buy dunlop absorbent grip and i want to know if i just put the overgrip over it or do i need to take the old one off?? Btw, i'm not sure how to take the old one off :S