How to pass from dark to bright in a video?

Hello,
I am starting from very low in terms of video knowledge. My situation is that I am trying to make an on/off video, in which I put my hand on the camera and when I remove it, I'm in another place and so on. So, when I remove my hand of the camera, it makes the video very bright due to the change from no light (when my hand is on the camera) to daylight. Is there any way to reduce this brightness?
I am using a Gopro.
Thank you

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