Upgrade OS DD6300 from 6.2.1.50 to 7.7.4.0

Hello 

My DD6300 has actually OS 6.2.1.50-680189 and I want to upgrade to 7.7.4.0 LTS.

I need to know if I can upgrade directly to 7.7.4.0 LTS or if there is an intermediate step or OS before upgrading to 7.7.4.0 LTS ? 

I need to know if I must apply the NDU before applying the 7.7.4.0-1017976.rpm ? 

If it is the case, what is the NDU that I must apply as I have actually the OS 6.2.1.50-680189 ? 

< >  or  < > ?

Please provide me the exact steps for this upgrade.

Regards

Hung