Help with updating existing Dell customised image VMware ESXi host to ESXi6.7U3

Hi,

Can anyone point me to any official Dell documented upgrade process to VMware ESXi6.7U3 please? 

There is plenty of documentation around installing 6.7 from scratch, but I can't seem to find anything regarding the correct/official process of upgrading an existing Dell ESXi host that is already running an official Dell Customised ESXi image - (R640 running ESXi6.0 in our case), up to ESXi6.7U3

(I'm already aware vCenter needs to be done first btw)

Thanks