Changing a thumbnail on a MP4 video

I am not experienced in video editing at all but I have an issue I need help with. I made a video (a friend screen shared and I recorded it for her) but we have an issue. Before we recorded the video she had a chat open, once we were ready she closed it and we started the video recording a minute later. For some reason though the thumbnail of the video is the chat. We need to change this as the chat is private and the video is supposed to be shared with others. How do I change the thumbnail though? I googled it and can't figure it out.

I am on PC, windows 10 and the video is a mp4.

Any help would be appreciated.