It sounds like you want to roll back the selenium version used rather than the Maven version. If this is the case, you just specify the version you want in your pom.xml where you request selenium. If this is done in a parent pom, you can copy that entry to your child pom which will override it.
If you want to rollback Maven, you have to install the version you want to another directory. (or delete 3.0.3 and then rollback)
I'm not quite sure what Selenium might be doing here, but the release:prepare goal is supposed to modify the release ID in the POM, verify that all your source code has been checked in, and tag the code in version control. If your VCS is SVN, that operation is atomic. Other systems, such as CVS could potentially fail with only part of the code tagged.
IIRC, there's a backup of the POM made when the POM is updated. For the most part, you should be able to rerun the goal, possibly with an override that Maven suggests. However, I think you've made Selenium unhappy, so there may be some stuff above and beyond your Maven stuff that needs repairing.
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