[Question] Mining Turtle Selected Block

Hi all,
Im just wanna find out if a turtle can detect a certain block that is the same in its selected slot and then break and replace it with a different block in a different slot?
Im stuck at this

If turtle.compareDown() then
turtle.digDown()
end
turtle.placeDown()
turtle.forward()

I dont know how to make it place a block other than the one in its selected slot

Lemme know
thanks

turtle.select(<slot>)

slot: from 1 to 9, like:
1 2 3
4 5 6
7 8 9

Thanks so much :3