Change default app to open attachments.in eM Client

could you please explain how to Change default app to open attachments.in eM Client

It is a Windows function.

Whatever app is set to open that file in the Windows OS, is what will open the attachment from eM Client. If you have not associated an app with that file type, you will get a popup from your OS asking to select an app to open it. If you want to change the default app to open that filetype, you will need to change it in Windows.