I need help with changing tile map tiles with code.

var tilePos = position + facingDirection * 16
tile_map.set_cell(tile_map.local_to_map(tilePos), )

Heres my current code. Basically this is some code to get the tile that is in front of the character and change it to another. I have it set up a little but I have no idea how I would get a reference to the tile I want to change the selected tile to.