It depends on whether your UEFI has a shell builtin. If it does, there should be an option in its settings / boot menu for you to launch it. Some motherboard also provide an option to launch a shell from the EFI system partition (ESP).
You should consult the manual of your motherboard for the path it will look for (the instructions are often vague, though). Usually they are looking for a file named Shell.efi in the ESP root folder.
Another way is to launch it just like you launch any other EFI binary (e.g., a bootloader). Since it's not really accessible to register an EFI binary in your UEFI or put the shell binary in your ESP in Windows, the easiest way is probably to add it as \EFI\Boot\bootx64.efi (also add the update.nsh script you need to run and the files it requires under *\EFI\Boot*) in a FAT(32)-formatted USB drive (it shouldn't matter whether it's MBR or GPT as long as your UEFI is standard-conforming enough). Then reboot and boot the USB in UEFI mode from your UEFI boot menu.
You can obtain the EFI shell binary from the EDK2 project repository:
P.S.: You may need to disable Secure Boot.