開いているバッファの文字コードを変更する

M-x set-buffer-file-coding-system<RET>
Coding system for saving file (default nil): s<TAB>

Click <mouse-2> on a completion to select it.
In this buffer, type RET to select the completion near point.

Possible completions are:
shift_jis			   shift_jis-dos
shift_jis-mac			   shift_jis-unix
sjis				   sjis-dos
sjis-mac			   sjis-unix

Coding system for saving file (default nil): sjis-dos<RET>

[参考]
With Emacs - 5.1 文字コードの変更