You can select the are you wish to export as WAV file using the start pointer and end pointer.

You can see these pointers on the ruler of both track editor and musical editor by clicking [Start Marker] and [End Marker] from "Transport Tool Bar".

Then, adjust the location of these pointer so that you will be able to assign the area of the export.

This can also be done by long pressing the left button of the mouse at the top green bar (the area above the ruler) which changes the cursor to the grab mode (you will see a hand grabbing something), and then drag & drop to adjust the location of the pointer.