Requirement:

1. Installed ADB and driver for your phone. Download ADB tools here, extract to a folder.

Step

1. Turn on USB Debugging on Developer Options
2. Plug your phone with USB cable to PC
3. Open windows explorer, go to extracted ADB folder and press "shift + right click", select "open command window here"
   
4. Make sure your phone detected by type "adb devices" and your phone number listed there.
   
5. Type "adb shell sm set-force-adoptable true" and then press enter
   
6. Unplug your phone from PC and go to setting > storage&usb > SDCard
7. Click 3 dot on top right corner
8. Setting > format as internal
9. Enjoy

Note

make sure you have backup your microSD card contents because this will reformat your card.