I have attached two MATLAB Apps that provide a highly simplified example of how to do what I think you are asking.
Please save these two files to your working directory.
To run the example type main on the command line.
This will open main.mlapp, and the startup function in main.mlapp will open helper.mlapp. (The second app may be right behind the other one so move the windows around to see both)
On the helper app, pick an animal name from the drop down, and then press the button to send the data to the main app.
Look at the startup function for main, you will see it calls helper, and passes a reference to itself to helper so that helper can send data back. A property, called mainApp is defined in help.mlapp to store this reference. When help.mlapp opens, it's start up function saves this reference to the property app.mainApp so that it can access it when it needs it.
The callback for the send push button then get the data from the dropdown assigning it to the local variable, selection, and then sends the data to the main app using app.mainApp.animalTable.Data = {selection}
