I pushed a controller button in PeriopSim and there is a popup menu showing

The Windows Mixed Reality menu can be displayed while running VR. Dismissing this menu can be done by a second button press.

Depending on which headset is being used, and which button was pressed will indicate which button to press again to return to PeriopSim.

Windows Mixed Reality Headset

If a row of 5 or 6 buttons is displayed on screen press the Windows button on the controller (small circular button located midway up the handle of the controller.

HTC VIVE

Press the power button on the controller (small circular button located bellow the trackpad) to return to PeriopSim.