If you don't have any plate properties in your model, you must create a new plate property before hand or create a new one by selecting the button off to the right.
Then select "Elem/Property Type" to change the type.
Then select "Plate" as your type.
You can now fill out the info of the new plate property.
I don't know if you can create a new property when you modify a property. I am sure that if exist other properties you can change the elements property to the new one.
I have see the previous post, you can create a new as suggested
Thank you so much Folks. I am assuming there will be an option to change from Membrane element without creating the property to the Plate elements. Other Pre-Processor do have this option