Ever came across a YouTube video where heads look like potatoes?
Then here comes your savior: www.stretch.site
Just paste the youtube link, and choose the correct switcher. You will usually want to use the 16:9 → 4:3 preset, as it is the most common mistake of greenhoppers that think it is "cool" to stretch a classic 4:3 video to 16:9 :-3
Enjoy!