How do I check the block below and replace it with another block?

I don't know how you want to accomplish this in game so I will invent something.

Create a new item call it whatever texture it whatever it does not matter.

Next go back into the item and create a new right click on block procedure.

Now the block code is llike this:

    if( block comparer [get block at x { y - 1 } z ] == minecraft: dirt ) {

       remove block at x { y - 1 } z

       place minecraft: stone at x { y - 1 } z

}

If statement can be found under the logic and loops category.

Block comparer can be found under logic operations it is the first yellow block with the cube icon only.

Minecraft: dirt and minecraf: stone can be found under minecraft components use the second yellow one with the white cube only.

Remove and place block can be found under block actions.

{ y - 1 } is a compound of y and the operator block in math - and the number one also found under math 1