If you need a high-quality external tool that doesn't require opening the Unity Editor, the EasySave3 Web Editor is the most prominent community-driven solution. :
While advanced users can install a JSON schema and manually decode ES3, this is tedious and error-prone. ES3 is not pure JSON; it uses tags like s3 and type attributes. Without a dedicated parser, you will almost certainly corrupt the file. es3 save editor high quality