How to install the latest version without vmlinuz file?

I want to upgrade my x86-64 based openwrt-19.07.7 into 20
In the manu page ,it says that:

OpenWrt on x86
For x86 systems there is no “sysupgrade” image, just be sure to use the new firmware image has the 
same family of filesystem (if the current firmware uses squashfs then the new will use squashfs as well 
and if the current has ext the new will use ext filesystem. Note that an upgrade from ext2 [10.03.1] to 
ext4 [12.09] seems not working. Tested 10.03.1 squashfs to 12.09 squashfs, working ; 10.03.1 squashfs 
to 12.09 ext4 failed; 10.03.1 ext2 to 12.09 ext4 failed)

So i want to install it.
Here is the way i install 19.07.7 version.

wget https://downloads.openwrt.org/releases/19.07.7/targets/x86/64/openwrt-19.07.7-x86-64-combined-ext4.img.gz
gunzip   openwrt-19.07.7-x86-64-combined-ext4.img.gz
sudo  wget https://downloads.openwrt.org/releases/19.07.7/targets/x86/64/openwrt-19.07.7-x86-64-vmlinuz -o  /boot/openwrt-19.07.7-x86-64-vmlinuz
sudo dd if=openwrt-x86-64-combined-ext4.img  of=/dev/sdb1 bs=4M; sync;

#Get all partuuid with sudo blkid and edit grub file.

sudo vim  /boot/grub/grub.cfg
menuentry "OpenWRT" {
        linux /boot/openwrt-19.07.7-x86-64-vmlinuz root=PARTUUID=5f8979fa-01 rootfstype=ext4 rootwait console=tty0 console=ttyS0,115200n8 noinitrd
}
menuentry "OpenWRT (failsafe)" {
        linux /boot/openwrt-19.07.7-x86-64-vmlinuz failsafe=true root=PARTUUID=5f8979fa-01 rootfstype=ext4 rootwait console=tty0 console=ttyS0,115200n8 noinitrd
}

Reboot my pc, 19.07.7 can be installed.
Now i want to install new version same way,it is simple to replace
openwrt-19.07.7-x86-64-combined-ext4.img.gz
with
generic-ext4-combined.img.gz
but in the page
https://downloads.openwrt.org/releases/21.02.0-rc3/targets/x86/64/
There is no vmlinuz file for booting 21.02.0-rc3!

How can do then?

You're looking at instructions from 2011.

Since then, the sysupgrade script has been improved to where you can just fetch the new combined image and run sysupgrade on it. It will parse out the necessary files and place them on your drive.

And how about to install the latest version from beginning with grub.cfg?