To update the attributes of a trunk, use the openstack network trunk set or openstack network trunk unset commands shown here:

openstack network trunk set
[--name <name>]
[--description <description>]
[--subport <port=,segmentation-type=,segmentation-id=>]
[--enable | --disable]
<trunk>

openstack network trunk unset --subport <subport> <trunk> 

The set and unset commands can be used to add and remove sub-ports from a trunk without impacting the running instance.