The conventional way of working out the aspect ratio of a wingletted wing is to add the area of the winglets to the wing area and their length (i.e. height) to the span.
So, in effect, the winglets are considered as if they have been bent downwards to act as extensions of the wings in the same plane. The APB winglet for the 737 Classic is around 7 feet tall, so the wingspan (for the purpose of calculating AR) will be about 14 feet more than an unwingletted aircraft, i.e. a tad over 33 metres.
Obviously taking the span as the distance from tip to tip of the upright winglets will give a different figure for the wingspan and therefore a different answer for the aspect ratio.