To do so on desktop versions, first, open up the WriteMapper app.
Then, select the WriteMapper → About WriteMapper option in the menu bar. The version number installed on your computer will be displayed in a new window that pops up.
On WriteMapper 2 for iPad or iPhone, you can first open a WriteMap, then from the mind map view, open the more menu (second button from the right in the toolbar), and select Help Menu... > About WriteMapper. The version you have will be displayed on the resulting dialog.