You don't provide details about which server you are doing this for nor what kind of datasource xml you are using but lets assume its a jboss server.

First you start by changing the password in the actual mysql database using mysql tools.

Then read the docs for how to encrypt passwords at Chapter 15. Encrypting Data Source Passwords

or something like stackoverflow article here: http://stackoverflow.com/questions/10520997/jboss-as-7-1-datasource-how-to-encrypt-password

hope that helps, but for future reference this forum is for help with eclipse developer tools, questions about how to use/configure JBoss is better left at forums dedicated to that.