To get the HSPL go Here
To Downgrade from the Mango Update's 4.3 Bootloader Go Here
It is a pretty easy and straightforward job for the Sprint Arrive, I cannot comment on any other versions. And you must check that you have only 4.3 (or 4.X where X = any number) but the downgrade will not work without the Goldcard method if anyone has a SPL Bootloader of 5.0 or above. Just be warned. If you brick your phone, it is your own fault that you could not read these simple instructions and warnings.
Read, be patient, and be rewarded.