Unfortunately, you can't. Only phones running "Marshmallow" can actually set an SD card to be "Internal" rather than the standard "External". If you can update your phone to "Marshmallow" you might get it to work. I say might because your phone may not have the capacity to allow for it.
To free up storage on the device I would suggest moving videos, photos, files and movable apps to the SD card. This will give you room to do more/have more on the device. Videos and Photos take up a lot of space.
Should you be able to upgrade to "Marshmallow" then you can use the following to try and make the card "Internal". However, please note, if successful, anything that was already on the SD card will no longer be there. When the card is made "Internal" it is formatted and encoded to work with only that device, so it can't be read by any other devices as well.
How to turn external SD card into internal storage on Marshmallow.
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 select “Format as internal”, and then “Erase & Format”.
4. Your SD Card will now be formatted as internal storage.
5. Reboot your phone.
NOTE: If you don't reboot the phone, many things may not work correctly, so make sure you do.
If you can't install "Marshmallow" your only other option would be to consider rooting the device. Should you choose to do so, I suggest you visit the "Rooting, Jailbreaking & Unlocking" Forum
http
/www.tomsguide.com/forum/forum-117.html.
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