On Microsoft Surface Pro
Wire will select the facing default camera to display videos when video calling. To switch from front to back camera and vice versa to make video calls:
- Select the Windows camera app.
- Switch the cameras.
- Return to Wire and start a video call.
Learn more in Microsoft Windows Surface Pro Tutorial
On mobile devices
While on a video call: Select the flip camera button 
to switch to the back camera. Select it again to switch back to the front camera.