I.e., gunzip the kernel and edit config.txt.

First, I edited /boot/config.txt and attempted to change the kernel line to the version that I had installed via apt as follows: kernel=vmlinuz-5.10.0-19-arm64
The "file" command reported both files (the previous kernel8.img and the vmlinuz-5.10.0-19-arm64) as "Linux kernel ARM64 boot executable Image, little-endian, 4K pages" -- so I thought it just might work.
When I rebooted, the system was stuck at the initial color "rainbow" screen.

I resized my boot partition (it was 128MB on the ascii image, but 256MB on the chimaera image).  I downloaded the file rpi-3-devuan-chimaera-5.10.110-arm64-ext4-2022-11-25.zip from the page https://arm-files.devuan.org/RaspberryPi%20Latest%20Builds/  ... then using dd I overwrote my boot partition with the one from that image.  At first it would not complete the boot due to missing partition (I forgot to edit cmdline.txt).  After I got that straightened out, it did boot into Linux devuan 5.10.110 #1 SMP PREEMPT Fri Nov 25 00:43:14 CET 2022 aarch64 GNU/Linux

So I guess that part is solved, sort of.  I was hoping there would be a simpler way to just upgrade the kernel (like it works in amd64).

Also, upgrading the kernel didn't even fix the iptables problem.

# iptables -A INPUT -s 1.2.3.4 -j DROP
iptables/1.8.7 Failed to initialize nft: Protocol not supported
# iptables -V
iptables/1.8.7 Failed to initialize nft: Protocol not supported

I think I've got the solution for this, though.
I found on https://superuser.com/questions/1480986/iptables-1-8-2-failed-to-initialize-nft-protocol-not-supported:

update-alternatives --set iptables /usr/sbin/iptables-legacy
update-alternatives --set ip6tables /usr/sbin/ip6tables-legacy

I had to copy the /lib/modules/5.10.110 from the latest image file to my live system, too (which of course I forgot to do previously).

Now, iptables seems to be functioning correctly.

The only question remaining is, whether or not there's a simpler kernel upgrade process.  At least things are working again, and it is possible to upgrade kernel, just a bit more time-consuming than I'm used to :-)

(Possibly there is some name limitations which would force you to copy your kernel+initrd into existing names)

The newest chimaera iso for rpi3 seem to have a file /boot/config.txt identifying its kernel, and that boot kernel is named kernel8. It is an gunzip of the accompanying vmlinuz-5.10.110.

Here's my situation:
I've got a raspberry pi 3b+.  I originally used the devuan ascii image on sd card, later moved to external usb drive.  After running this setup for a few years, I finally (today) decided to dist-upgrade to beowulf, and then to chimaera.  It all went pretty smoothly.  The one issue that I've noticed during startup now is that iptables refuses to load the rules I have stored in a script.  An example rule is: iptables -A INPUT -s 1.2.3.4 -j DROP and the message that was returned on beowulf is: iptables: Invalid argument. Run `dmesg' for more information..  dmesg contains nothing useful or related to the iptables command.  On chimaera the error is: iptables v1.8.7 (nf_tables): Could not fetch rule set generation id: Invalid argument.

From some internet searching, I believe that the problem is due to an outdated kernel.  uname -a result is: Linux devuan 4.16.14-v8+ #1 SMP PREEMPT Tue Jun 5 18:50:10 CEST 2018 aarch64 GNU/Linux.

I then tried: apt install linux-image-arm64 which proceeded to download and install linux-image-5.10.0-19-arm64_5.10.149-2_arm64.deb
This did create the files in /boot, but they were not loaded upon a system reboot, it's still on 4.16.14-v8+.

I *believe* (but may be mistaken) that the newer rpi images contain a newer kernel.  Could I download the chimaera image, and copy the contents of its /boot to my /boot in order to load a newer kernel?  Or is there another way to perform a kernel upgrade on a running system?  Any help is greatly appreciated.