This one "The Breathing App" is free and simple and has a very helpful information section as well as links to helpful information videos. Good support for creating good Coherent breathing  habits. Developed by yoga teacher Eddie Stern and Deepak Chopra.  Most people need to set it to 5:5  or 6:6 (in for 6 seconds, out for 6 seconds), or if that’s too slow then start at 4:4 or whatever starting rate works for you and gradually work towards 5:5 or 6:6. Don't put too much pressure on yourself, as ANY breathing rate that is slowing you down towards the Coherent Breathing rate is going to support you to feel better.