To reset that setpoint you need to get moving. Even if you continue to eat normally, but add exercise to your daily living, you will lose weight. Exercise burns calories, so even if your metabolism has slowed down, the activity will help you burn more calories. Exercise can give your metabolism a short term and long term boost. It combats the effects of the slowed-down metabolism from the lower caloric intake.
Weight training is also good for burning calories.
Studies have shown that weight training can help a woman burn an extra 300 calories a day, at rest. The strength training workout itself burns about 500 calories an hour. But don't rush into a full scale workout. Your body will soon adjust to that and slow down at that speed. Bump it up a bit in increments.