var lay_id = layer_get_id("Tiles");
var map_id = layer_tilemap_get_id(lay_id);
var mx = tilemap_get_cell_x_at_pixel(map_id, 224,256);
var my = tilemap_get_cell_y_at_pixel(map_id, 224,256);
var data = tilemap_get(map_id, mx, my);
var ind = tile_get_index(data);
data = tile_set_index(data, 4);
tilemap_set(map_id, data, mx, my);