You need Odin3 v3.07 or newer (Windows) to flash a custom recovery first. For Mac users, use Heimdall.
Before you begin, ensure you have the following:
Before you search for that “upd” file, prepare the following:
On some third-party firmware archive sites, you might see this update labeled as:
This is the most critical step. You cannot install Android 5.1.1 using the stock recovery. You need a custom recovery like TWRP (Team Win Recovery Project) or ClockworkMod (CWM) .