I'm a little surprised that the opener is still working. Garage door openers usually don't have the power to open the door without the assistance of the spring but you probably have two springs and just one has broken. Even though the opener is working, it's a good idea to replace the spring to reduce wear/tear on the opener.

Can you replace a torsion spring? Sure. Should you replace a torsion spring? Maybe. When wound, a torsion spring contains a tremendous amount of energy that can be very dangerous if you make a mistake. If you want to do the work yourself, I would strongly suggest reading up on the subject. I have no affiliation but there's a company called DDM Garage Doors that has lots of good information on the subject. Scroll down to the part on replacing torsion springs.