this is kind of work in progress and not perfect for all scenarios but as synology now offers 6.2.4 in the DSM GUI as update (instead of 6.2.3 U3) more people might have that problem, some people tested that already here (https://xpenology.com/forum/topic/41473-dsm-624-25554-install-experience/) but i wanted to continue in a easier to find area and try to sort out problems and fine tune here in the tutorial and guides section
there is the more complicated way of going back to 6.2.3, keeping all settings and data, that needs some experience in general and some linux skills and a easier way where you keep your data but loose the settings (user, shares, ...), the skills needed for this is more or less the same what you need to install xpenology, a link to the less complex way is added down below as a PS
download a rescue/live linux like system rescue cd (i used a "older" version 6.0.3, mdadm and lvm worked ootb, any newer should be fine too), transfer it to a usb (not your dsm boot usb) to boot from it
assemble your raid1 system partition like here (1st partitions of all disks as /dev/md0)
skip anything about swap or volume1 data partition, we only need access to the dsm system partition https://xpenology.com/forum/topic/7004-tutorial-how-to-access-dsms-data-system-partitions/
mount the assembled raid1 to /mnt with this
mount /dev/md0 /mnt
then remove some files with this
rm -rf /mnt/SynoUpgradePackages
rm -f /mnt/SynoUpgrade.tar
rm -f /mnt/SynoUpgradeindex.txz
rm -f /mnt/SynoUpgradeSynohdpackImg.txz
rm -f /mnt/checksum.syno
rm -f /mnt/.syno/patch/*
also check the files containing the DSM Version
cat /etc/VERSION
cat /etc.defaults/VERSION
if the one in /etc shows 6.2.3 and the one in /etc.defaults shows 6.2.4 delete or rename one with 6.2.4 and copy the one with 6.2.3 to that place (a version file with slightly more content is also in the *.pat file that can be opened with 7zip, that can be used for comparison too)
report say's its working too if you just delete both VERSION files
shutdown the rescue linux
shutdown -h now
now you will have to restore the kernel files on your boot usb to 6.2.3 (the update also replace files on the loaders 2nd partition)
win10 can have some difficulties with mounting the 2nd partitions of the loader, so look here https://xpenology.com/forum/topic/29872-tutorial-mount-boot-stick-partitions-in-windows-edit-grubcfg-add-extralzma/
(it can also be done with linux but i have not tried what other tools will extract the kernel files but if you are familiar with linux you will find out
https://xpenology.com/forum/topic/25833-tutorial-use-linux-to-create-bootable-xpenology-usb/)
on 2nd partition delete all files except extra.lzma and extra2.lzma (if its 3615/3617 then its just extra.lzma)
use 7zip to open "DSM_xxxxx_25426.pat" (dsm 6.2.3 install file, depends on you dsm type 3615/3617/918+)
extract "rd.gz" and "zImage" and copy it to the 2nd partition of your xpenology usb
the assumption here is you already had 6.2.3 before the 6.2.4 update try, if not you would use the kernel files from the *.pat of the 6.2.x you had installed
as a example what could go wrong: if you had 6.2.2 and did 6.2.4 your extra/extra2.lzma on the loader where usually replaced with special ones made for 6.2.2, if you add 6.2.3 kernel files to the 6.2.2 extra/extra2 (drivers) then most of the drivers will not work as of the incompatibility with 6.2.1/6.2.2 with 6.2.0/6.2.3
if you are unsure about the extra/extra2 or you dsm version then use the original extra/extra2 from jun's loader (img file can be opened with 7zip, loaders kernel files and drivers in its extra/extra2 are 6.2.0 level) or use a extended version of the extra/extra2 made for 6.2.3 and use the kernel files of 6.2.3 if you had a older 6.2.x that would do a update to 6.2.3
put back your usb to the xpenology system, boot up, find it in network (i used synology assistant) and "migrate" to version 6.2.3 (keeps data and settings) - dont try internet that will do 6.2.4 again, use manual install and give it the 6.2.3 *.pat file (you already have as you needed it to restore the kernel files on the loader)
it will boot two times, one for 6.2.3, 2nd for 6.2.3_U3 (it will be downloaded automatically if internet connection is present)
everything should be back to normal except patches like nvme ssd patch (or other stuff you patched after installing 6.2.3 that is not dsm update resistant)
oh and one more thing - check your update settings, it should look like this
please comment on how to make it easier to follow, its just a short version i tried once
PS:
if that sounds all to complicated then its still possible to use the other downgrade method (but you will loose all settings and end with a factory default DSM - but you data volume(s) would still be unchanged)
https://xpenology.com/forum/topic/12778-tutorial-how-to-downgrade-from-62-to-61-recovering-a-bricked-system/