Question

if epidermal cells wear out and are replaced everyday then y do we get tanned due to sun exposure?

Answer 1

it's not about epidermal cells. it's about the increase of melanin pigment which gives the natural colour of skin. in order to protect the skin from sun light, melanin pigments increase and skin colour becomes darker.

dark coloured people have more melanin pigments than light-coloured people. this is an adaptation against sun exposure. when you look at the places where dark coloured people live, you'll see that those places receive much more sunlight than the places that light coloured people live.

Answer 2

also, it's not like you wake up with a different skin everyday :-) the pigmented cells lie below the dead layer that partly falls off over time. these dead cells are colourless, allowing UV-rays to penetrate deeper into the skin.