How can i change the sprite image of button in run time of a 2d game

i am trying to make a 2d game in unity.And every thing is ready except for 1 logic .I want my player which is currently a circle should change its shape according to the selection and i want to know to know that is it possible to change the sprite image of button in unity on runtime. Because the collider i am using is polygon collider so it would adjuct itself accodingly. Every other logic like buying logic and other UI layers are selected and this is the final logic my game is missing please help me.