Disabling Secure Boot on XPS 13 7390

It's not clear why you want to disable secure boot -- but if it's to install second OS (say, Linux), note that the "Advanced mode" you're looking for -- which is probably legacy boot/CSM Option ROM -- is not supported by this model.  This is a UEFI-only model that does not support legacy booting, so whatever you are doing will require a UEFI-complaint medium and OS or utility.  

@ejn63 I already successfully installed GNU/Linux on this machine, it is perfectly fine to boot it using UEFI, what I need to do is to disable the Secure Boot.

The reason why I want to do that is that i need to load the vboxdrv kernel module to be able to run virtualbox on this machine. I know that I could sign the modules, but since Secure Boot is completely unusefull to me and I should be able to disable it, I just want to disable it.

@ejn63  There are several Linux distros that natively support UEFI booting, without requiring CSM/Legacy Option ROMs, but do not have a bootloader signed by Microsoft that would pass Secure Boot verification in its default state.  So in those cases you either have to disable Secure Boot (without also having to enable legacy options) or customize your system's trust store so that it includes the certificate of whoever signed your Linux distro's bootloader, in which case it would be trusted.

@ecalore What is the exact warning or error message you see?  If you're stuck even though you have that system in front of you, then other people you ask for help who don't have that system right in front of them won't be as likely to be able to help if you just give a vague description of what you're seeing rather than providing as much information as you can.

@ecalore  Ok, I've never seen the XPS 13 7390's BIOS interface myself, so I can't really fill in the blanks here.  One thing you could try is pressing F12 during startup to access the one-time boot menu.  On the Dell systems I have, one of the options in there is "Change BIOS Boot Mode" or something like that.  And in there you can choose between:

Legacy Boot, Secure Boot Off
UEFI Boot, Secure Boot Off (which also enables CSM/Legacy OROMs)
UEFI Boot, Secure Boot On

You might not have that first option, but you might have an easily accessible menu option to disable Secure Boot there.  Otherwise, based on what you described there's probably some other setting in a General section of the BIOS Setup that allows you to choose whether to show Basic or Advanced options throughout the interface or something.

You can then use the setting you indicate to toggle secure boot off.  The message may be there about "advanced" mode if the system were locked down with an administrative password;  it's also entirely possible the same BIOS core is used on another system where that option requires more than standard access -- only the engineer who wrote it probably knows.

@jphughanunfortunately I do not have the machine in front of me, I am offering help to a colleague remotely.

Anyhow, in the machine service manual it is just written to find the "Secure boot" tab in the BIOS menu and there to switch it from Enable to Disable.

I can easily find this "Secure Boot" tab in the BIOS menu, but there I find just a message saying that to toggle that switch I need to enable the BIOS Setup "Advanced Mode", I am looking for some hints about where this "Advanced Mode" can be enabled.

p.s.
Do not know why bothering the users with this... If I am in the "Secure Boot" tab to change it, why to ask me to go somewhere else to do an additional step to allow me to change it.

The answer is quite clear. due to criticality and time crunches on failure, trying to create clones and images of Computers that if they fail OS wise we can bring them back up very quickly . We have WIN 10 Pros that are brand new and cant seem to do this. Although we have viable WIN XP and Win 7 machines that have images. This is the nature of our particular business we need it because if it goes down and we dont have it we could be down for weeks 

@roblv  Did you perhaps reply in the wrong thread here?  The post by ejn63 you replied to is over 9 months old, and your reply about needing to image systems doesn't directly relate to the topic in this thread about having trouble disabling Secure Boot.  If you're using Windows 10 images, you should be able to set them up so they boot in UEFI mode and therefore would support Secure Boot, in which case the issue that was discussed here shouldn't be blocking you from creating images.  If this issue somehow DOES block you from creating or using images, then it might be worthwhile to provide some additional background that others here might be able to use to help you troubleshoot your particular issue.