This has been a problem of those using an android phone that is running android 6.0 Marshmallow, though those running android 5.1 lollipop are still facing the same problem but this tutorial is just and only for andriod 6.0.
Make sure you have an SD card that has enough memory, like 16gb SD card.
Then follow the bellow steps
1. Turn OFF your phone>>insert the SD card>>Turn it ON again
You will get a notification that a new SD card has been detected
Now double tap the notification
3. Tick Use as internal storage then click on Next
4. Before you implement this step, make sure you backed up all your file in a safe place or phone memory.
5. Click Erase and Format (Wait while the SD card get formatted)
6. Tick Move now or Move later depending on when you wish to move media files from the internal storage to the newly prepared SD card
7. Click Done and reboot your your device.
Once the process is completed, any app you download from any source will be automatically installed into your SD card as default memory.
NOTE: Once you format your external SD card as internal storage on Marshmallow, it can only be used on the same device then. To use it on other devices, a format will be required, which ensures the safety of your data on the SD card.