Wait until the cheese has started to melt before flipping so that the melted cheese holds the ingredients in the quesadilla better. If possible, flip the quesadilla over the folded edge to help prevent the chicken from falling out. However, if you are able to gently slide a wide spatula under the quesadilla, then quickly flip it over (in any direction) you should be just fine. It may take a little practice, but anything that falls out of the quesadilla (which happens to everyone) can just be quickly tucked back in with the tip of the spatula.