(continue)
Now you can boot your machine from the first stick we prepared, wait for SBUU to run, select Firmware update and then connect the 2nd USB Stick (the one we just prepared with repository manager
(Note: don't insert this stick before running SBUU).
Alternative Method: Using Just Repository Manager
There's an alternative method, but it is not really clear.
As I said, it ended up saying the the updated weren't compatibile with my system, but, despite that message, the system has been updated.
The procedure for this is in the video posted by Rey: en.community.dell.com/.../1767.dell-repository-manager.aspx