First find the revision numbers for the file(s) in question.

Use the log command to get the info from previous commits including revision numbers.

So something like…

svn log update-data.php 
--------------------------------------------------------------
r4057 | eoin | 2010-03-25 17:33:32 +0000 (Thu, 25 Mar 2010) | 1 line

turn on db errors
--------------------------------------------------------------
r4056 | eoin | 2010-03-25 17:32:36 +0000 (Thu, 25 Mar 2010) | 1 line

add update file
--------------------------------------------------------------

Use diff command to see the difference between the 2 revisions.

svn diff -r 4056:4057
Index: update-data.php
=============================================
--- update-data.php	(revision 4056)
+++ update-data.php	(revision 4057)
@@ -6,7 +6,7 @@
 
-$db->show_errors = false;
+$db->show_errors = true;

Sometimes this is all you need, as you can see the mistake you made. Now you can make a change to the working copy and commit a fix. However if there were too many changes to do yourself, you may prefer to revert to the last known good revision. In this case, you will need to merge the changes from your current revision back to the revision you want to revert to.

You should try a dry run to see what a the outcome of a merge will be before you execute the real merge.

svn merge --dry-run -r 4057:4056 update-data.php 
U    update-data.php

Now execute the merge command to revert to previous revision.

svn merge -r 4057:4056 update-data.php 
U    update-data.php

svn diff update-data.php 
Index: update-data.php
==============================================
--- update-data.php	(revision 4057)
+++ update-data.php	(working copy)
@@ -6,7 +6,7 @@
 
-$db->show_errors = true;
+$db->show_errors = false;

Use a diff to see the changes made after the merge and you should see that the change(s) made have been undone. Now commit the working copy to include this change and all’s well in the world again.

svn -m 'turn off db errors again' commit update-data.php

One final point, be super careful with a merge, things can get real messy!
Take your time, make sure you use diff to see what changes have been made and check again when your through

Really good and detailed explanation of the revert process from aralbalkan.