If you found this on a forum, look for a provided MD5 or SHA-1 hash to ensure the file hasn't been tampered with.
This is a non-official update typically found on file-sharing sites or community forums. Official GOG updates are usually delivered via GOG GALAXY or standalone installers from your GOG account library . Stellaris.Update.Only.v3.5.3.to.v3.6.0.1.GOG.rar
Upload the file to VirusTotal to check it against dozens of antivirus engines. If you found this on a forum, look
Community updates often require you to overwrite files in your main Stellaris directory. Ensure your base version is exactly v3.5.3 ; applying this to other versions will likely cause the game to crash. Stellaris.Update.Only.v3.5.3.to.v3.6.0.1.GOG.rar