Use System Restore in windows and see if it has a restore point before you installed Windows 8.1, not sure if it allows rollback tho.
or
With the laptop being that age i doubt you’ve got backup disks, it should however have a hidden backup partition.
Backup all your files, My Documents, Pictures, Videos, Music, Desktop etc…
Reboot and computer and try and get into the recovery partition, quite often its Alt-F10 but a quick google for your laptop brand should tell you.
Run a full recovery and it will wipe your laptop to how it was when you took it out the box.