DoMagic

function collect()
    turtle.select(2)
    for i = 1, 4 do
        for i = 1, 3 do
            turtle.digDown()
            if i < 3 then
                turtle.forward()
            end
        end
        turtle.turnRight()
    end
    turtle.dropUp()
end

    function replace()
    turtle.select(1)
    for i = 1, 4 do
        for i = 1, 3 do
            turtle.placeDown()
            if i < 3 then
                turtle.forward()
            end
        end
        turtle.turnRight()
    end
end

while true do
    turtle.select(1)
    if not turtle.compareDown() then
        turtle.select(16)
        if turtle.compareDown() then
            collect()
            replace()
        end
    end
end