Manually set post featured photo/post thumbnail

Hello,

I want to set up my WP installations so if my client forgets to include a featured photo for the post (aka post thumbnail) then I would find the first image in the post and set that as the featured item.

I know how to extract the image data form the post content, but how do you manually set the metadata so it uses said extracted image as the post’s featured photo/thumbnail?

Thanks 🙂