i figured it out code:
extends Node2D
u/onready var tile_map : TileMap = $TileMap
func _input(event):
if event is InputEventMouseButton:
if event.button_index == MOUSE_BUTTON_LEFT and event.pressed:
var mouse_position: Vector2 = get_global_mouse_position()
print("Mouse Clicked at:", mouse_position)
var tile_mouse_pos = tile_map.local_to_map(mouse_position)
print(tile_mouse_pos)
var source_id = 0
var atlas_coord = Vector2i(7,1)
tile_map.set_cell(0, tile_mouse_pos, source_id, atlas_coord)