Are you using this for a or a general research paper ? Knowing the context can help me find the exact version you need.
Go to GitHub and search for "neptune.cls" or "neptune-latex" . neptune.cls download
If you only need it for one paper, simply place the neptune.cls file in the as your .tex main file.When you write \documentclass{neptune} at the top of your document, LaTeX will look in the local folder first. Option B: The Permanent Way (System-wide) Are you using this for a or a general research paper
Ensure the filename is exactly neptune.cls (all lowercase) and that it is in the same directory as your project file. Error: Missing Packages neptune.cls download