If you’re no longer happy with the person you’ve named as a power of attorney or the powers you gave them in that document, it’s not enough to renounce that verbally. You need to take the extra step to clarify what this will look like by destroying the former document and creating a new one while also updating anyone who knew about the previous arrangement. 
You have the right to revoke an existing power of attorney at any time, but it’s recommended that you work with an estate planning lawyer to do so. Your local estate planning lawyer can give you clarity around the action steps you took and give you peace of mind that you’ve done the necessary steps to revoke this document properly.
Make a statement in writing about your intention to formally revoke the old document so there is no confusion. When you right this, make sure to state that you are of sound mind and understand the implications of revoking this old document. You should mention the name of the original agent and the date this other document was executed.
Send a copy to the old agent and any institutions that had this on file so that you can fully protect the revocation, especially if you are executing a new POA document. More questions about crafting or revoking a POA document, sit down with your lawyer and discuss your next steps.