Check with your vet to meke sure she is healthy. Split her food into two meals and reduce the total daily intake by an eighth. Give her no extras at all. Up her excersise time and encourage her to run when she is off the lead. If she isn't losing weight in two weeks reduce her food again by an eighth. When she has lost the weight increase her food by very small amounts every week until she is on an even keel and stick to it.