Cannot update operating system from El Capitan to High Sierra

I have a macbook ( mid 2010 13"). The operating system was updated a couple of years ago from Snow leopard to El Capitan ( yes I know, I am a dinosaur). I have tried unsuccessfully to update the system today- in App Store using Update tab it is stating NO UPDATES AVAILABLE. I have checked the check boxes and these are activated to accept updates. I found out that the latest operating system comparable with this model is High Sierra. The newest system Catalina is not available for this very elderly machine!

I have no idea how to access this as no updates are available. Any bright ideas very happily received. As I am not the most switched on technically, I really struggle with this sort of thing. Thanks folks!