On the Start menu, select Settings > Apps > Default apps.
Select which default you want to set, and then choose the app. You can also get new apps in Microsoft Store. Apps need to be installed before you can set them as the default.
You may want your .pdf files, or email, or music to automatically open using an app other than the one provided by Microsoft. To choose default apps by file type. scroll down and select Choose default apps by file type.