I am going to go out on a limb and assume you have a native connection running between Marketo and Microsoft Dynamics. When you go to the Admin section in Marketo and click on Microsoft Dynamics under Integrations you will find the area where your MSD credentials are entered. I am however concerned if my assumption is right and this is about your integration app for the native integration. If only your client secret changed you should be able to update that, but a connection once made is almost impossible to change, so I am wondering why that secret key would have changed.
Please provide us with more information so we can be of more help indeed.
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