It depends on a library manifest (library.json, library.properties or module.json). If library's author updates "version" field, then PIO Library Crawler will update it in a registry.

Please note that you can manually update manifest and make PR to author's repository. Click on "Manifest" tab when looking at a library in Web Registry and you will see "Edit" button.

Like ikravets said, the crawler just scrapes Github, so updating the libs is just a pull request away. As a library creator, I like how integrated it is with Github, which means I don't need to upload a new version of my library anywhere, I just need to remember to change the version in the JSON.