How to "save with encoding"

Recently there is a plugin written for this: Convert-file-encoding

To use the plugin, press Ctrl+Shift+P, type Convert File Encoding to search for the function and select the encoding you want the file convert into.

Another option, without using any plugin: cut all text from file, then switch to the target encoding (Ctrl-Shift-U) in an empty editor and paste the text back.

As seen in https://github.com/atom/encoding-selector/issues/7#issuecomment-274257654