The best we can do with this information is derive an expression for the potential energy change:
One of the challenging aspects of potential energy is that their is no absolute location that is designated zero potential. However, this is also one of the most useful aspects about it. One is free the set any location as being zero. All that can be measured are changes in the potential energy of a system.
So, when the present problem refers to "a reference level", this is just another way of saying that we will set that level as zero and calculate the change in potential energy that occurs when we move the object from that point to some other point. (Potential energy changes always involve rearranging objects in the presence of a conservative force such as gravity or the electric force.)
So, with all that said, here is the answer:
Because your problem has not specified what the change in altitude is, we must leave the answer in this form.