You have your OS installed in you laptop. If not make sure you have any OS installed may be windows. And you must have a copy of OS may be in DVD or an ISO image burnt in a USB disk.
Now start you laptop plugged in charger with minimum charge uptp 70%.
Attach the disk and start the setup.
Go as per instructions.
Done. You will be prompted at the start up to chose desired OS to be loaded
You can now switch between OS any time.