Probably best to just risk it and do the upgrade. Even on a slow machine it should only take 30 minutes to process.
Fedora keeps 3 kernels installed so once if you fail to boot the f42 one you should still be able to use your old known good f40 one.
But if that good one is already at the bottom of the list you should either uninstall the middle two or increase the saved amount by setting installonly_limit=5 in /etc/dnf/dnf.conf
If it does fail during the upgrade it will likely break, but no actual data (especially in /home) would be lost. If it fails during the final cleanup/verifying step then it shouldn’t have any impact.
You can also use this helper afterwards to verify all package files are in–tact: Brace/brace/usr/sbin/brace-rpm-verify at 18e41d34229e21921f18dbcd83bad692ee487ed7 · divestedcg/Brace · GitHub
As for not having storage for a backup: I’ve always enjoyed dumpster diving for goodies and even if a laptop or whatever is damaged their disks have always been plentiful for me.
There are also some free cloud options that you could use to backup:
- Oracle Cloud Always Free tier offers 200GB of space and you could just use encrypted restic or borg to it
- if you’re in a college/uni, you may have a large space available as part of Google/Microsoft offerings
Also if you’re at a larger workplace with lots of computers maybe drop by the IT department and ask if they have any drives there were going to toss/recycle
If you really do want to put it off further then see my usual spiel: Security comparison - GNOME and KDE Plasma - #12 by SkewedZeppelin
edit: if you want to get creative you can also give this a try: Make use of Btrfs snapshots to upgrade Fedora Linux with easy fallback - Fedora Magazine