You can't do that directly. You would need to install an email client/app on your computer and you could then download the email onto your computer where it would be saved.
Presumably you don't at present have an email client installed and if you don't want to install one you could open the email you want to save then go to the "More" button in the menu bar and click on it and scroll down to "Save as EML File. This will save the email into your download folder.
The EML file can be opened by an email client/app but if as previously stated you don't have an email client/app installed you could then rename EML file to MHT and double click on it. MHT format is compatible with EML and MHT files could be opened by any web browser.
If all you are wanting is a copy of the email you could print it and then scan it and store it as a file on your computer.
By far the easiest way is to use an email client/app.
