public void Modify( 
   DomainDescription domainDescription
)

public void ModifyDomain(Geodatabase geodatabase, string codedValueDomainName = "Pipe")
{
    SchemaBuilder schemaBuilder = new SchemaBuilder(geodatabase);
    CodedValueDomain codedValueDomain = geodatabase.GetDomains().First(f => f.GetName().Equals(codedValueDomainName)) as CodedValueDomain;
    CodedValueDomainDescription codedValueDomainDescription = new CodedValueDomainDescription(codedValueDomain);

    // Update domain description
    codedValueDomainDescription.Description = "Water Pipe Domain";

    // Adding code/value pair
    codedValueDomainDescription.CodedValuePairs.Add("C", "Copper");

    schemaBuilder.Modify(codedValueDomainDescription);

    // To modify the orders of coded value domain
    // schemaBuilder.Modify(codedValueDomainDescription,SortBy.Name,SortOrder.Ascending);

    schemaBuilder.Build();
}