There are two options when cooking a whole chicken breast side down: Either leave it like that during the entire roasting process or flip it over after roughly thirty minutes to allow the breasts to brown. Both methods work but you will have to remove the chicken from the oven in order to flip it, which will cause your oven to cool down a bit in the process. The benefit of taking it out and turning it over is that the skin on top will still have the opportunity to get nice and crispy.
If you choose to leave it as is, not only will the oven stay at a consistent temperature, but the white meat will be a little juicer than if you turn it over partway through. And while the breast skin won’t come out crispy, the skin on the other side of the chicken will crisp up instead. It can also be a bit messy to flip. Ultimately a matter of preference and totally up to you which way you do it.