Note, that this tutorial will work on other devices too, but you wouldn't find the Firmware in this tutorial, so before starting ask Google for you Firmware.
Watch the Video tutorial from bottom of the article or read the Written tutorial.
1. Install USB driver for Samsung SPH-L720T on your computer.
Download Samsung Galaxy S4 driver here
P.S. The drivers that Windows automatically installed doesn't fit our requirements, so you better install those from the link above.
2. Activate USB Debugging from Settings > Developer options > USB debugging, if you don't find Developer options under the System tab, you need to activate it.
How to activate Developer options on Samsung
SPH-L720T
Navigate to Settings > About device > tab Build number several times till a warning message appear stating: Developer options activated
3. On your computer run Odin3, make sure that Auto Reboot and F. Reset Time are checked. In other Odin3 versions this options are under the Options tab.
Download Odin3 here
Download Android 5.0.1 Marshmallow ROM for Samsung Galaxy S4 SPH-L720T Sprint
5. Start your phone in Download Mode.
For this turn off the phone, then press and hold Volume Down + Home + Power keys. Hold them till a warning message appears. Then you need to confirm that you want do download a custom OS pressing Volume up key.
7. Press the START button and wait till the process finishes. During the process its normal for the phone to restart several times.
That's all.
