Example below is grabbing the request from your TestCase and assigning a value to a specific element.
// get XMLHolder for request message def
def groovyUtils = new com.eviware.soapui.support.GroovyUtils(context);
// grabbing the specified request
def holder = groovyUtils.getXmlHolder("Specified#Request")
holder["//*:Password"] = 'password1';
For the example above you need to know the Xpath for your element.
Note, that this can be accomplished in several ways, but you specified doing in through groovy script. It can also be done through a #TestCase# property. Example:
<soapenv:Body>
<tns:AuthenticateUser>
<tns:Credentials>
<tns:IntegrationID>${IntegrationID}</tns:IntegrationID>
<tns:Username>${Username}</tns:Username>
<tns:Password>${Password}</tns:Password>
</tns:Credentials>
</tns:AuthenticateUser>