As of Terrier v4.0, Terrier ships with Hadoop 0.20.x. Terrier 4.2 will upgrade to a more recent Terrier version ( TR-316). This wiki page provides the latest documentation on trying Terrier with different Hadoop versions.
Key Message: Hadoop can be changed by replacing the jar files in the lib/hadoop0.20 folder, and adjusting your CLASSPATH to your Hadoop installation. YMMV.