HELP! Reaching 6.0 with firmware 5.1

I'm currently using a m9 Sprint/nTelos with firmware 5.1 , but I can not upgrade to 6.0 because no exist that firmware for this model.
I tried to install custom ROM based on 6.0 but no start obv my firmware is 5.1.

there a way or a rom to reach 6.0 with firmare 5.1 ?

acctually using MaximusHD 6.0

I'll be very greatful!!!

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