You can remove a number of instances of the model by executing maax_deploy.sh with remove as the -mo argument. For example, the following removes one instance of the dga model, version 1.0:

$METRON_HOME/bin/maas_deploy.sh -zq node1:2181 -mo REMOVE -m 512 -n dga -v 1.0 -ni 1

If you need to modify a model, you need to modify the model itself and deploy a new version, then remove the old version instances afterward.