If you are using Ubuntu you can use EFI boot manager.
sudo install efibootmgr
sudo efibootmgr
BootCurrent: 0000
Timeout: 10 seconds
BootOrder: 0000,0004,2001,2002,2003
Boot0000* ubuntu
Boot0003* Windows Boot Manager
Boot0004* Windows Boot Manager
Boot2001* EFI USB Device
Boot2002* EFI DVD/CDROM
Boot2003* EFI Network
The boot order is the list, and if you want to delete ubuntu here, which is zero, just ask efibootmgr to remove it from the boot order.
sudo efibootmgr -o 4,2001,2002,2003
and if you want it back
sudo efibootmgr -o 0,4,2001,2002,2003
if you want to boot windows by default
sudo efibootmgr -o 4,0,2001,2002,2003
Edited: If you don't have access to ubuntu, try booting from a bootable Ubuntu via USB. Use the latest 18.04, it should have efibootmgr in the desktop live ISO.
Additional reading if you want to do this in windows, see microsoft documentation: https://docs.microsoft.com/en-us/windows-hardware/drivers/devtest/editing-boot-options-in-efi