Flashing this kind of software to your device will require you to already have either CWM or TWRP recovery installed, doing so will also force you to unlock your device's bootloader, and again your device's warranty will be voided as the unavoidable consequence.
But in regard of testing and finding the best Android Nougat based custom for your Samsung Galaxy S4 LTE then there is no wrong for taking the risk. But of course the choice is yours to take. You are the decision maker of what to do to your device. My part is only sharing the guides. Okey?
- Make backups before starting the guides.
- Don't forget to enable your device USB debugging mode.
- Be sure you follow all the steps written on this post and don't get sleepy and skip any steps.
- Prepare your computer and a USB cable, if you don't have ones, borrow from friends.
- Gapps are included in the ROM zip file.
How to update Samsung Galaxy S4 LTE I9505 to Android 6.0.1 Norma N7 Custom ROM Tutorials
Your warranty will be voided. Perform the guides written on the post and all the risks are yours to take.
- Download Android 7.1 Substratum ROM to your PC.
- Now you need to use your USB cable to connect your device to PC. Once they're connected then transfer all the zip files to your device's SD card.
- Now power off your Samsung Galaxy S4 LTE I9505 then unplug it from USB cable.
- Now enter to the Recovery Mode / TWRP mode.
- Inside the TWRP mode, perform the following action full data wipe by choosing the Wipe button -> Swipe to Factory Reset button.
- Now back to the TWRP main menu then you need to tap the Install button. And then choose the ROM zip file you previously transferred to your device's storage on step 2.
- Now confirm the installation by simply swiping the Swipe to Confirm Flash option. You will see the installation will begin.
- Once the installation has been completed then repeat the above action to flash the Gapps.
- Now you need to tap Wipe cache/dalvik button in case you experience bootloop or other technical problem.
- Now you need to tap the Reboot System button.