You can go to Settings, then Details, and go to Default contact preferences settings. You can change the default communication preferences for all clients. Use this setting with caution as it will override all individual client preferences.

For example, if you set SMS & Email, then all new clients will have their communication preferences set to this. You always have the ability to update the setting individually for clients.

You can click apply this setting to existing clients and override all existing preferences settings for clients.