This is quoted from this blog
You just committed a change to the repository and now you realize it was a mistake; how do you back your changes out in the working copy and recommit the corrected version?
Easy; use the merge subcommand which can create and apply a diff between two revisions.
Say the oops code lies in version 198 and you want to back your changes out to version 197, then use merge like so:
svn merge -r198:197 myfile.java
Notice how the is from 198 to 197, so the changes will be backed out.
Now commit the corrected version back in to the repository:
svn commit -m ‘Backed out that nasty divide by zero…’