You can install the latest Eclipse plugin version from the update site https://javahexeditor.sourceforge.io/update or the Eclipse Marketplace https://marketplace.eclipse.org/content/java-hex-editor. Older versions of the Eclipse plugin are available via the update site for the version, e.g. https://javahexeditor.sourceforge.io/update/0.5.1
You can download the latest stand-alone version and older versions on the "Files" tab.