Here is what I did
1. stop cassandra service with: service dse stop
2. open up /etc/dse/cassandra/cassandra.yaml and change the 'initial_token'
3. restart cassandra with: service dse start
But I don't see the change takes effect, nodetool still shows the old token. I guess there must be some steps I missed, especially after reading http://wiki.apache.org/cassandra/FAQ#clustername_mismatch