hi non,
here is how:
<packet version="1.5.0.0">
<domain>
<set>
<filter>
<id>[domain_id]</id>
</filter>
<values>
<hosting>
<vrt_hst>
<property>
<name>ftp_password</name>
<value>[new password]</value>
</property>
<ip_address>[IP address of the server this user is on]</ip_address>
</vrt_hst>
</hosting>
</values>
</set>
</domain>
</packet>
it kills me that nobody helps each other on this API on these forums, as if no one has implemented it... you can't even find a trace of it.
I'm going to publish it open source though just a bit left.