To move from using the classic schema.xml in your opensolr index, to the managed-schema simply follow the steps below:

In your solrconfig.xml, look for a SchemaFactory definition, and replace it with this snippet:

<schemaFactory class="ManagedIndexSchemaFactory">
    <bool name="mutable">true</bool>
    <str name="managedSchemaResourceName">managed-schema</str>
  </schemaFactory>

If you don't have any schemaFactory definition, just paste the above snippet to your solrconfig.xml file, just about any requestHandler definition.