I-93 to Lincoln, then follow signs to Rte 112 (east), Kancamagus Highway, or Conway.
Note that Conway, on the eastern end of the Highway, is often very crowded with auto traffic and commercial development. Avoid or take West Side Road if heading north towards North Conway, Crawford Notch or Mt Washington. Or simply make a U-turn before Conway and enjoy the Kancamagus again back to Lincoln. Along the eastern portion of "the Kanc", look for the junction of Bear Notch Road. This side road offers a more intimate view of the White Mountains and ends at Bartlett and route 302. Bonus: route 302 north goes through scenic Crawford Notch.
I cheated. In a most years, it is difficult not to find good fall color along the extraordinarily scenic Kancamagus Highway (Rte 112) running through the heart of the White Mountains between Lincoln and Conway.