Do you mean you would like to change the default IM application for Office?
If yes, this setting is controlled by registry.
Please refer to this link: How Office integrates with an IM client application
Path: HKEY_CURRENT_USER\Software\IM Providers
You would see a value named DefaultIMApp which you can modify.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.