So, I was playing with my APU1, and thought I'd try it myself ...
The starting point is a SD card with a snapshot image written, and I added an additional 21.02.01 to the same SD card, then rebooted into 21.02.01.
This is all done from the already running snapshot version
note: with the default size of /dev/sda1, you will only be able to fit three kernels.
you'll need:
opkg install resize2fs fdisk blkid
create new partition for the new openwrt version, I used 250MB.
fdisk /dev/sda
Command (m for help): n
Partition type
p primary (2 primary, 0 extended, 2 free)
e extended (container for logical partitions)
Select (default p): p
Partition number (3,4, default 3): 3
First sector (33280-15523839, default 247808):
Last sector, +/-sectors or +/-size{K,M,G,T,P} (247808-15523839, default 15523839): +250M
Created a new partition 3 of type 'Linux' and of size 250 MiB.
Command (m for help): p
Disk /dev/sda: 7.4 GiB, 7948206080 bytes, 15523840 sectors
Disk model: Card Reader
Geometry: 245 heads, 62 sectors/track, 1021 cylinders
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0x4958be76
Device Boot Start End Sectors Size Id Type
/dev/sda1 * 512 33279 32768 16M 83 Linux
/dev/sda2 33792 246783 212992 104M 83 Linux
/dev/sda3 247808 759807 512000 250M 83 Linux
Command (m for help): w
The partition table has been altered.
Syncing disks.
wget and write the new openwrts ext4 root FS of the new partition, resize it to fit the partition size, and check it.
cd /tmp
wget https://downloads.openwrt.org/releases/21.02.1/targets/x86/64/openwrt-21.02.1-x86-64-generic-ext4-rootfs.img.gz
gzip -d -c openwrt-21.02.1-x86-64-generic-ext4-rootfs.img.gz | dd of=/dev/sda3
root@OpenWrt:~# resize2fs /dev/sda3
resize2fs 1.45.6 (20-Mar-2020)
Resizing the filesystem on /dev/sda3 to 64000 (4k) blocks.
The filesystem on /dev/sda3 is now 64000 (4k) blocks long.
root@OpenWrt:~# fsck.ext4 /dev/sda3
e2fsck 1.45.6 (20-Mar-2020)
rootfs: clean, 1347/13312 files, 4856/64000 blocks
DL the new version kernel, and store it in /boot
wget https://downloads.openwrt.org/releases/21.02.1/targets/x86/64/openwrt-21.02.1-x86-64-generic-kernel.bin -O /boot/vmlinuz-21.02.1
get the PARTUUID of the new partition
blkid /dev/sda3
/dev/sda3: LABEL="rootfs" UUID="ff313567-e9f1-5a5d-9895-3ba130b4a864" BLOCK_SIZE="4096" TYPE="ext4" PARTUUID="4958be76-03"
add a new menu entry to grub, by cloning the existing one
vi /boot/grub/grub.conf
replace the kernel file name (default vmlinuz) with the one you used when you wget it, and update the
PARTUUID with the one from the blkid command.
menuentry "OpenWrt 21.02.1" {
linux /boot/vmlinuz-21.02.1 root=PARTUUID=4958be76-03 rootwait console=tty0 console=ttyS0,115200n8 noinitrd
}
in grub.conf the set default="0" controls which menu entry in the config file will be booted by default,
if noone interrupts the countdown. 0 is obviously the 1st entry. The timeout="2" is the count down
timer, in sec.
reboot, and keep fingers crossed 