There's no API specifically for regenerating group enrollment keys. However, you can use the CreateOrUpdateEnrollmentGroupAsync method to update an existing enrollment group, passing in a new set of keys. See: https://learn.microsoft.com/en-us/dotnet/api/microsoft.azure.devices.provisioning.service.provisioningserviceclient.createorupdateenrollmentgroupasync?view=azure-dotnet&viewFallbackFrom=azure-dotnet-preview You will need to generate your new symmetric keys to pass in as part of the EnrollmentGroup parameter.
The following sample shows an example of using this method with an enrollment group that uses X.509 certs, but you should be able to easily modify it to use symmetric keys instead: https://github.com/Azure/azure-iot-sdk-csharp/tree/main/provisioning/service/samples/getting%20started/EnrollmentGroupSample