UpdateReleaseDefinition.
        [Preview API] Update a release definition.
        :param :class:`<ReleaseDefinition> <azure.devops.v5_1.release.models.ReleaseDefinition>` release_definition: Release definition object to update.
        :param str project: Project ID or project name
        :rtype: :class:`<ReleaseDefinition> <azure.devops.v5_1.release.models.ReleaseDefinition>`

def update_release_definition(self, release_definition, project):
        """UpdateReleaseDefinition.
        [Preview API] Update a release definition.
        :param :class:`<ReleaseDefinition> <azure.devops.v5_1.release.models.ReleaseDefinition>` release_definition: Release definition object to update.
        :param str project: Project ID or project name
        :rtype: :class:`<ReleaseDefinition> <azure.devops.v5_1.release.models.ReleaseDefinition>`
        """
        route_values = {}
        if project is not None:
            route_values['project'] = self._serialize.url('project', project, 'str')
        content = self._serialize.body(release_definition, 'ReleaseDefinition')
        response = self._send(http_method='PUT',
                              location_id='d8f96f24-8ea7-4cb6-baab-2df8fc515665',
                              version='5.1-preview.3',
                              route_values=route_values,
                              content=content)
        return self._deserialize('ReleaseDefinition', response)