In this scenario, the demand for oranges would increase. This would lead, all other things being equal, to an increase in the price of oranges.
When lemons were declared unhealthy, the demand for them would drop. Consumers would then switch to oranges since they are (in this question at least) a substitute. This would mean that the demand for oranges would rise. When demand rises the price of the product rises unless there is an increase in supply to compensate for the increased demand.
Therefore, in this scenario, the demand for oranges would rise, leading to an increase in their price.