I am a dual system installed with UEFI+GPT. Computer: Lenovo R720 System: Windows10 +Ubuntu16.04
After installing Ubuntu16.04, Win10 from the GRUB menu was somehow missing. (If you don’t even have GRUB on display, click on Windows 10 to install Ubuntu.)
Goal: Access Win10 from Ubuntu GRUB.
Implementation:
The terminal executes the following commands in turn

sudo grub-install /dev/sda
sudo chmod -x /etc/grub.d/30_os-prober
sudo gedit /etc/grub.d/40_custom

Enter the following in the 40_custom file that opens:

menuentry 'Win10' {
insmod part_gpt
insmod chain
insmod ntfs
set root='(hd0,gpt1)'
chainloader /EFI/Microsoft/Boot/bootmgfw.efi
}

The terminal continues to execute sudo update-grub
The file /boot/grub/grub.cfg has been changed.
After rebooting, the Win10 option appears in the GRUB menu.
More:
If need to modify the order of the grub or modify grub background:
this is grub Customizer installation method: https://www.tuziang.com/combat/635.html