Default How do i upgrade to the new 4.4 bootloader from 4.3?

I need to know what the difference is between 4.3 and 4.4. How do i upgrade to it? Do i need to do it to run kitkat roms?

I have flashed about 4 kitkat roms and they all have issues. Like freezing and rebooting. I think maybe because i am on the 4.3 bootloader.

I just dont fully understand this stuff yet.

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