Plugin update won't upload to my website

I’m not sure what this “update” to Table Rate shipping is about. Don’t plugins update through my control panel? Why would I need to download it again from Envato if it’s just an update? I tried to upload the zip file as a plugin to my website, and my control panel said the “destination file already exists” and “upload failed”. Is this some sort of ploy to get me to buy some support? I certainly don’t want to delete the version I have working on my site now…cuz then I have to reconfigure all the settings, right? So what am I supposed to do with this file I downloaded?

I can see others have asked similar questions, and the answer has been “You shouldn’t lose all your settings.” So I “could lose them all, but I shouldn’t?!” Why is this paid-for plugin so horribly complicated when it comes to updating itself. Why can’t it update like every other plugin?