For these sort of operations, I would recommened to use Fields class from the Tridion practice wiki: https://github.com/TridionPractice/tridion-practice/tree/74f53c5b611347d09100be12c644566488974b80/ChangeContentOrMetadata

For example:

    var component = (ComponentData)client.Read(tcmuri, DEFAULT_READ_OPTIONS);
    var fields= Fields.ForContentOf(schemaFields, component);

   
    fields["richTextField"].Value = "some rich text content";
    component .Content = fields.ToString();

    client.CheckOut(component.Id, true, DEFAULT_READ_OPTIONS);                       
    client.Save(component, DEFAULT_READ_OPTIONS);
    client.CheckIn(component.Id, DEFAULT_READ_OPTIONS);