Using BTRFS we gain access to some pretty useful features such as snapshots of the filesystem. By default we exclude the home and log directories so only the relevant system parts get saved. If you run into problems or bad upgrades you can always boot into an older snapshot (which gets created everytime the system gets updated, keeping you safe) using GRUB and then restoring the older snapshot.
A rough overview of the procedure (a video showing the whole process is available as well)
If the latest snapshot is not working try the older ones.
Restoring Using Terminal or tty
Note: Type Ctrl + Alt + {F1,F2,F3,F4,F5,F6} to go to TTY. TTY is very useful when you can't log in graphically.
1. Type sudo timeshift --restore
in your terminal or TTY.
2. Choose the date and time to restore. Each timeshift snapshot corresponds to a number, type this number to choose the snapshot.
3. Confirm the restoration by pressing ENTER.