I unrooted my phone to get OTA update but while updating in the boot screen, it stopped at around 25% with the red triangle and the Android bot then reboots automatically.
I have tried many times with no luck.
How should I go about this?
Since rooting changes the system image, the OTA update won't apply. You need to download the ROM (e.g. from sammobile.com) and flash it by hand (using Kies).