How to do HIIT on a stationary bike?

I was told a stationary bike is the most low impact to do HIIT & I would love to try it. I never did before because I have issues with my knees & didn't want to risk getting an injury. Anyhow, I know the basic idea is to go all out for 20 seconds, slow for 40 seconds & repeat. But, do you leave the resistance the same through out or do you change it? Any information on how to do this correctly will be greatly appreciated. Thanks!