How to convert all images in webp and delete the other images

There are a few plugins for that, like https://wordpress.org/plugins/imagify/ and https://wordpress.org/plugins/webp-converter-for-media/

However, I recommend waiting a bit, maybe a year, as only the more recent versions of browsers support WebP, and most notably Internet Explorer does not and never will since it was discontinued: https://caniuse.com/webp