I take it that your father is no longer competent to sign the beneficiary designation form himself. Whether or not you can change his beneficiary using his durable power of attorney will depend on the insurance company rules. You will have to contact the insurer and ask them.
I’m also not quite sure what you mean by “dual.” If it means that you and someone else are both appointed on the durable power of attorney, then whether you can act alone will depend on both the wording of the actual power of attorney and, again, on what the insurance company requires.
For more information about your duties as an agent under a power of attorney, check out this article.