Pack a bit of everything. Yesterday here I wore shorts it was sunny and warm. Today windy and 54. You will probably get a mix of weather, some days start out cold and end up very comfortable.
This is the time of year when most of the natives begin to "layer" up so we can adapt as the day goes along. Andover is in the foothills of the Western Maine mountains area, so will be cooler than some lower-elevation, closer-to-the-coast area. They've already had their first overnight frost warning of the year. I'd anticipate daytime high in the upper-60's-to-low-70's at warmest and overnight lows in the 40's or so -- with possible exceptions to either. A pair of medium-weight slacks, llght blouse with long-sleeved sweater-or-fleece to go over it plus a windbreaker or light raincoat should get you by in most situations and allow you to add or subtract layers as needed from day-to-day or during the day.
Maine = Casual. Flannel shirts, fleece jackets, jeans, wooly socks and sturdy shoes or boots. Rain is always a possibility as is 80's and sunny! Think Land's End or LLBean if you want to just blend in... If you are going out for a nice meal, business casual is good 95% of the time. Enjoy your trip!