How to add deletion policy in existing policy?

[Mark_S (Baldrick)]

I have a simple rotating policy and I want to add a deletion phase. This is not available in the UI, however I have done by creating a policy directly with PUT, including the delete section.
My question is if it is possible to update an existing policy
I tried to PUT only the delete part, but it was rejected.
In Create or update lifecycle policy API | Elasticsearch Guide [8.6] | Elastic I see that the whole policy is given - is it not possible to only PUT the new part?

Here is my policy:

{
  "testindexpolicy" : {
    "version" : 1,
    "modified_date" : "2023-03-17T12:08:04.639Z",
    "policy" : {
      "phases" : {
        "hot" : {
          "min_age" : "0ms",
          "actions" : {
            "set_priority" : {
              "priority" : 100
            },
            "rollover" : {
              "max_primary_shard_size" : "1gb",
              "max_age" : "1d"
            }
          }
        }
      }
    },
    "in_use_by" : {
      "indices" : [index_from_logstash_test_alias-000002 ],
      "data_streams" : [ ],
      "composable_templates" : [
        "test_template"
      ]
    }