Consider you have a file that uses improper encoding on special characters. Example: You see the latin1 version "ñ" for the UTF-8 "ñ" but the file itself is stored as UTF-8 (meaning that the UTF-8 bytes are doubly encoded).

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You can fix that easily with Vim:

vim broken.file

Now you tell vim that the file's encoding is actually latin1 (you can see what Vim is currently using by saying only :set fileencoding):

:set fileencoding=latin1

Write and reload the file:

:w
:e

All should be good now.

Adjust to your needs for any other encodings.