To rollback the system to it's previous state, reboot and choose desired target snapshot in the grub boot menu.
Test the snapshot, and if the system works fine, rollback by creating a writable snapshot:
sudo snapper --ambit classic rollback
Reboot again and choose the default grub entry. Due to the limitations of vanilla grub distribution (/boot/grub/x86_64-efi cannot reside on a btrfs subvolume), it's necessary to reinstall the bootloader after the reboot:
sudo grub-install --target=x86_64-efi --efi-directory=/boot/efi --bootloader-id=debian
More info: