I the regular version of the game, hit the "H" key. that is the default key to release the mouse.
In VR, there is a white ball that floats in front of you, that is your cursor. Moving your head moves the ball/curser. Point the ball at what you want to click and then click the left mouse button. Ive programmed a couple of buttons on my joystick to use as "left mouse" and "right mouse" buttons as finding the mouse on my desk with a headset on is a pian in the backside.
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