Please don't type in all caps.
Well, without "Marshmallow" the options are very limited and, to be honest, even if you did have "Marshmallow" considering the manufacturer, it probably still wouldn't work to change the SD card into "Internal" storage. They often block this ability on their devices.
Your best bet is going to be first, move photos, videos, music, etc. (not apps) to the SD card. The fastest way to do that would be to connect phone to a computer via USB cord and then move all the files you want on the card over to an empty folder (or a few folders) on the computer. Then, via the same USB connection, copy them all back over tot he SD card. I say copy as this will leave them on the computer as well, as a backup. Backing up, always a good idea.
After that you should also consider making sure apps are actually closed when you aren't using them. Having them open and going all the time in the background can add a lot of data to the phone.
I would also suggest that you install an app for cleaning. Me I like "Clean Master" but there are many out there.