Do it gradually. A sudden change can be stressful so do a little at a time. I changed my boys from the All Living Things hamster and gerbil mix to Hazel Hamster over the course of a week. I started by giving a tiny bit of Hazel Hamster with the all living things and gradually increased the amount of new food and decreased the amount of old food until I was no longer feeding the old mix
If you have anymore questions don't hesitate to ask!