What's the Kubernetes API to update annotation to API server

Hello experts:

We used below delete() to delete annoation,

	annotation := r.FdbCluster.ObjectMeta.Annotations
		if _, ok := annotation[fdbv1.BackupTrigger]; ok {
			//Remove post restore annotation
			delete(annotation, fdbv1.BackupTrigger)

then need to update the value in API server, so used update(), but it will cause other parts of spec changes which is not expected, we want to only update the annotation part, not other spec part.
From the update() method

// Update implements client.Client.
func (c *client) Update(ctx context.Context, obj Object, opts ...UpdateOption) error {
	defer c.resetGroupVersionKind(obj, obj.GetObjectKind().GroupVersionKind())
	switch obj.(type) {
	case *unstructured.Unstructured:
		return c.unstructuredClient.Update(ctx, obj, opts...)
	case *metav1.PartialObjectMetadata:
		return fmt.Errorf("cannot update using only metadata -- did you mean to patch?")
		return c.typedClient.Update(ctx, obj, opts...)

It sounds update() method doesn’t support update annotation. so what API should I use? patch()? any examples are highly appreciated. thank you.
ps: below is the k8s client version we used.

||k8s.io/client-go v0.21.3|
||sigs.k8s.io/controller-runtime v0.9.6|