al_replace_path_component
void al_replace_path_component(ALLEGRO_PATH *path, int i, const char *s)
Replace the i'th directory component by another string. If the index is negative then count from the right, i.e. -1 refers to the last path component. It is an error to pass an index which is out of bounds.
See also: al_insert_path_component, al_remove_path_component
Examples: ex_path_test