1. Install and run
External 2 Internal SD (Root) (the phone has to be rooted). That will swap internal and external storage.
2. Update to Marshmallow (at least) and format the SD card as adoptable storage. You'll lose "internal" and "external" and have just "storage". The phone will decide what goes where.
(Downloads and app installations
should go to internal storage - it's how Android works. External storage - SD cards - was designed to keep media files.)
3. Use an app like
Total Commander to copy files in internal storage, go to the SD card and paste the file (one file at a time).
Any of the 3 ways will work, depending on what you want to end up with. (With #2, you can't move the SD card to a computer and "see" the files - the format is completely different than any computer can read.).