Can't boot into Ubuntu in Windows 10 / Ubuntu dual boot

I installed Ubuntu on a Dell XPS 13 laptop that had Windows 10 preinstalled on it. I installed it from a USB flash drive on a partition.
When I boot my computer I can only boot into Windows 10 and Ubuntu is nowhere to be seen.

If I boot into my USB then I can see that Ubuntu is installed, but I can't get to it from the BIOS boot menu.

Disable the hibernation mode and Fast Boot in Windows.
Open command prompt as administrator and execute :

powercfg /h off  

Open the legacy version of the Windows Control Panel (not the modern version).
Select Energy Settings, enable show hidden settings and uncheck Fast Boot.
After having done this - shutdown the computer completely - do NOT reboot.
Boot into the BIOS and select Ubuntu as the default operating system to boot.

Update :

In case there is no Ubuntu entry to be found in the BIOS / UEFI settings,
re-install the GRUB boot loader to your Ubuntu installation in EFI mode.
Boot from the Ubuntu installation media - open a terminal and execute:

sudo mount /dev/sdXXX /mnt
sudo mount /dev/sdXX /mnt/boot/efi
for i in /dev /dev/pts /proc /sys /run; do sudo mount -B $i /mnt$i; done
sudo chroot /mnt
grub-install /dev/sdX
update-grub  

Note:

sdX = disk | sdXX = efi partition | sdXXX = system partition

To identify the partition numbers use GParted - it is included in the Ubuntu installation media.
Boot into BIOS and select Ubuntu in UEFI settings to be the default operating system to boot.