You just do the reverse from what you did to make the external card be the default. Just make sure you move all your files and apps back to the phone first. If you don't they will be inaccessible.
1. Go to device “Settings”, then select “Storage”.
2. Select your "SD Card", then tap the “three-dot menu“ (top-right), now select “Settings” from in there.
3. Now you want to set it to be "External" again, and then choose “Erase & Format”.
4. Your SD Card should now be "External" storage again.
5. Reboot your phone.
NOTE: If you don't reboot the phone, many things may not work correctly, so make sure you do.