Everything is in the directory tree at /boot
For example, on my phone ls -1F output
boot.scr
boot.scr.bak
config-6.5.0-1-librem5
config-6.6.0-1-librem5
dtb@
dtb-6.5.0-1-librem5@
dtb-6.6.0-1-librem5@
dtbs/
grub/
initrd.img-6.5.0-1-librem5
initrd.img-6.6.0-1-librem5
lost+found/
System.map-6.5.0-1-librem5
System.map-6.6.0-1-librem5
vmlinuz-6.5.0-1-librem5
vmlinuz-6.6.0-1-librem5
So you can see that there are basically two sets of files, one for 6.6.0-1 and one for 6.5.0-1
and the higher numbered one is what I am actually running and the other one is what I was previously running prior to the most recent kernel update.
However I think that you will not be able to change the kernel from the host computer. I have never done it but I think you would need to run Jumpdrive on the phone, then telnet into the phone from the host computer, and then use the flash-kernel command. (Exact command line is in this forum.)
And for me df /boot says that the boot partition is 45% full, which is satisfactory.
I think @FranklyFlawless was just suggesting that you clone the whole disk before the reflash i.e. using Jumpdrive create a disk image of the entire current contents of the phone’s disk to a disk image file on the host computer. That way you can retrieve any file that you need at any time in the future.
It may depend on what information you are specifically seeking. Does APN refer to MMS or just to general mobile network connection? If the former, @FranklyFlawless pointed you at the file. If the latter then /etc/NetworkManager/system-connections is a directory that will contain the required config file.