Flipping a quesadilla can be a bit tricky, especially if you're working with a heavy filling. To prevent spillage, make sure you're using a large enough pan and don't over-fill the quesadilla. To flip, use a spatula to gently lift one edge of the quesadilla, then slide it under the entire quesadilla. Hold onto the spatula with one hand, and use your other hand if needed to gently flip the quesadilla over. If you are really having trouble, try adding a lid over your pan for 30-45 seconds to help the cheese melt onto the top tortilla and keep it in place when you flip it.