get the latest version of the delegate for your harness account and update the image that the delegate upgrader uses

custom_delegate_upgrader_image.py

from requests import get, put

if __name__ == "__main__":

    harness_account_id = "wlgELJ0TTre5aZhzpt8gVA"
    harness_platform_api_key = "xxxxxxxxxx"

    custom_delegate_location = "my_docker_repo/harness/delegate"

    resp = get(
        f'https://app.harness.io/ng/api/delegate-setup/latest-supported-version?accountIdentifier={harness_account_id}',
        headers={
            'x-api-key': harness_platform_api_key
        }
    )

    resp.raise_for_status()

    latest = resp.json().get("resource", {}).get("latestSupportedVersion")

    full_path = f"{custom_delegate_location}:{latest}"

    print(f"setting to {full_path}")

    resp = put(
        f'https://app.harness.io/ng/api/delegate-setup/override-delegate-tag?accountIdentifier={harness_account_id}&delegateTag={full_path}',
        headers={
            'x-api-key': harness_platform_api_key
        }
    )

    resp.raise_for_status()

    print(resp.text)

The only permissions needed for the API key used in this script is delegate: edit at the account level.