In the best-case scenario, you’ll never need to defend against a knife attack ever in your life. However, having the skills to defend yourself and never needing them is far better than needing the skills and not having them.
The most important thing is to RUN when you have the chance.
In order for a knife to damage you, the person holding the knife has to catch up to you and make contact with you. Stay light on your feet and maintain your distance, run away if possible.
Place barriers between you and the attacker.
A barrier is anything the threat must avoid or move around in order to get to you. For example, go around a park bench or a vehicle, or move a chair or shopping cart.
Protect your vitals
If contact is made, do what you can to protect your vital organs and arteries (On the sides of your neck, armpits, groin, and inside of your thighs).
How to fight back
If you have no choice but to fight back, focus on controlling both arms of the attack, not just the hand controlling the weapon. Reason being that the attacker can switch which hand the blade is in and catch you off guard.
Don’t forget that the attacker is still human and is still vulnerable to attacks. Once you’ve controlled the weapon, you may want to finish the fight by knocking out the attacker with hard strikes.
Remember that if you’ve never trained to fight before, you will likely not know how to control someone or strike effectively, meaning fighting is not really an option for you.
Furthermore, you will get cut. However, getting cut and surviving is better than getting stabbed to death.