Download Samsung USB driver.
Download Firmware For Samsung Galaxy Tab A 2016 SM-P585 Marshmallow 6.0.1 Update Firmware.
Install the Samsung USB Driver to your PC.
Extract the Odin and firmware Files in your PC.
Copy the Unikernel files to your PC.
Switch off your Mobile.Enter into Download mode->Volume Down+Home+Power Buttons.
After entering into Download Mode it will say Warning Press Volume Up button to continue.
Connect Your Mobile With PC via USB Cable.
Launch Odin Application.It Will Show the message added.If you did not get this message Repeat the above steps.
After Getting Added Message Select PDA Button then Browse for the Firmware file selects it.Tick the Auto reboot.
Press Start wait for the tasks to complete.It may enter into Recovery mode to add the SuperSu permissions.Reboot again.
You can Check the Firmware Details->Settings->About->Version.
The Update Includes Marshmallow Update.
Google Now on Tap.
Improved Boot Security.
Improved Touch Wiz UI.
Important Security Patch Updates.