To set a default email client in Windows 10, you can do the following:

1. Select the Start button, then the Settings icon.
2. Select Apps.
3. Select Default apps from the left menu.
4. Under Email, select the current default program.
5. Choose a new app.
6. Select the X to close the default apps screen.

To set your default email app on a Mac, you can do the following:

1. Open the Mail app
2. Make sure the app is set up to use an email account
3. Select Settings or Preferences from the Mail menu in the menu bar
4. Click General
5. Select an email app from the Default email reader pop-up menu
6. If you don't see your email app in the menu, select 'Select' from the menu, then select your email app from the file dialogue that opens