Here's how it's calculated: www.sparkpeople.com/resource/calorie_calcu
It is recalculated any time you hit the Save button on the weight goals page, and on the fitness goals page. If you want to have your numbers redone, check and save both of those pages.
You can manually set different nutritional goals, but it's easiest to change Spark's range by changing your goal date. Bring it forward if you want a lower range, push it out if you want a higher one.