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 here: Microsoft Windows Surface Pro Tutorial
On mobile devices:
While on a video call: Tap 
underneath the self preview to switch to the back camera. Tap it again to switch back to the front camera.