Switch up her diet and make sure she's not over fed. Excercise her often and slowly bring up the work load slowly if she isn't used to it. Exercise is the most important thing though. Walk her in the walker and frequently ride. Good luck!!
Make sure she's not being over fed, and give her regular exercise. For losing weight I love to add 45-60 mins of walking in the walker in the morning to the training schedule. Walking or trotting for a long time on a hack, a lot of canter transitions help with stamina and weight loss. If she's very much overweight always consult with your vet before making big changes to her training or feeding schedule. Good luck!