If you have made extensive changes to a file and then change your mind about them, you can get rid of all changes by reading in the previous version of the file. To do this, use M-x revert-buffer, which operates on the current buffer. Since reverting a buffer can result in very extensive changes, you must confirm it with yes.
You may request that revert-buffer check for an auto-save file
that is more recent than the visited file by providing a prefix
argument. If a recent auto-save file exists, revert-buffer
offers to read the auto-save file instead of the visited file
(see Auto Save). Emacs asks you about the auto-save file before the
request for confirmation of the revert-buffer operation, and
demands y or n as an answer. If you have started to type
yes to confirm the revert operation, the y will answer the
question about using the auto-save file, but the es will not be
valid confirmation for the reversion. This gives you a chance to cancel
the operation with C-g and try again with the answers you really
intend.
revert-buffer preserves the value of point (in characters from
the beginning of the file). If the file was edited only slightly, you
will be at approximately the same piece of text after reverting as
before. If you have made more extensive changes, after reversion point
may be in a totally different context than your last edits before
reversion.
A buffer reverted from its visited file is marked “not modified” until
you make a change. The buffer’s modes will also be recalculated, by
normal-mode.
Some kinds of buffers whose contents reflect data bases other than files,
such as Dired buffers, can also be reverted. For them, reverting means
refreshing their contents from the appropriate data. Buffers created
randomly with C-x b cannot be reverted; revert-buffer
reports an error when asked to do so.