CRM 2016 How can edit body message email template by Plugin

How can to edit a E-mail template Body by plugin in CRM 2016?

the Template it is already exist ,I retrieve the Template ID by code plugin , and i want edit the message body by plugin.

to retrieve the message body email template i use with 'description' attribute. and if i want to update the body email template with this attribute ,with 'description', this updated the description box not the body message.

The following code describes receiving an e-mail template, how can update the message body by this Template?

//=============get template by name==================

private Entity GetTemplateByName(IOrganizationService client, string templateName) { var query = new QueryExpression();

        query.EntityName ="template";

        var filter = new FilterExpression();

        var condition1 = new ConditionExpression("title", ConditionOperator.Equal, new object[] { templateName });

        filter.AddCondition(condition1);

        query.Criteria = filter;

        EntityCollection allTemplates = client.RetrieveMultiple(query);

        Entity emailTemplate = null;

        if (allTemplates.Entities.Count > 0)            {

        emailTemplate = allTemplates.Entities[0];

        }

        return emailTemplate;
    }        

Thanks