Oracle Database Migration Assistant for Unicode (DMU) is a unique next-generation migration tool providing an end-to-end solution for migrating your databases from legacy encodings to Unicode and that too with maximum data integrity and efficiency.
Traditionally Oracle Oracles character set migration utilities, CSSCAN & CSALTER were used.
The links to the patch downloads (depending on the version) are on the Support website at Doc Id #1272374.1: The Database Migration Assistant for Unicode (DMU) Tool
Documentation for DMU can be found here