I assume that you meaning adding/removing classes from a host. This is undocumented in v1, but you can pass and array of puppetclass ids to the parameter puppetclass_ids in the json data on your PUT call.

"{\"puppetclass_ids\":[12,30,45,77]}

I am working on a more RESTful and documented way in v2.

Actually, the code is already in v2 that you can use but since API v2 is still in development, the docs haven't been released you.

To use API v2, just pass version=2 in the header

curl -u admin:secret -H 'Accept:application/json,version=2'

and POST to hosts/:id/puppetclass_ids with parameter :puppetclass_id as the id to be added

Here is the code at
https://github.com/theforeman/foreman/blob/develop/app/controllers/api/v2/host_classes_controller.rb

api :POST, "/hosts/:host_id/puppetclass_ids", "Add a puppetclass to host" 
      param :host_id, String, :required => true, :desc => "id of host" 
      param :puppetclass_id, String, :required => true, :desc => "id of puppetclass"

def create
        @host_class = HostClass.create!(:host_id => host_id, :puppetclass_id => params[:puppetclass_id].to_i)
        render :json => {:host_id => @host_class.host_id, :puppetclass_id => @host_class.puppetclass_id}
      end