You can't make the SD card "Internal" storage on a device with a version of Android that is older. Sadly you do need at least "Marshmallow" or newer. Unless, of course, you were to root the device. Which personally I do not recommend as it can totally and easily brick your phone.
What you can do is move all apps that are movable to the card. Also save your photos, files, videos, music, etc. to the card rather than the phone. You should clear out your history and cache for things like your browser and other apps often. Unless it is something you absolutely need, most people keep way to much on the phone. If you want to say save texts, don't leave them in the text messaging app. Save them to say email and on the SD card, or to a computer. The same goes for any other files or data.
Not only does all that take up space, but if the phone dies, gets lost or stolen, or the SD card gets damaged, etc., you would lose all of that. So it is smart to keep things you want to 'keep' off the device and off the card.