how to recalibrate battery

iOS performs the calibration. There is no user-provided control for that function.

If your battery is reporting calibration issues, or a health of 80% or lower, contact Apple Support and discuss your options and alternatives with them.

Per Apple: Learn more about getting service for your battery

Recalibration is not a good idea.

Typically it involves draining the battery to nothing, and then recharging it.

HOWEVER, Li-Ion batteries degrade quickly when they are drained to zero. Repeatedly draining is one of the fastest ways to destroy a Li-Ion battery.

I did a "Recalibration" to a Li-Ion device years ago, and it never came back to 100% because of that. I tend to avoid totally draining any of my Li-Ion devices.

TheRealMcCoy93 wrote:

Let it die out charge it up to 100% don’t take it off the charger at 100% let it sit for a couple more hours use your device again and repeat process after it chargers up to 100% the next time power off and back on

This is no longer a recommended procedure. Years ago, it used to be helpful in re-calibrating the battery sensor (you can't really re-calibrate the battery) so that you got the most accurate indication of how much charge you had. As MrHoffman explained above, this is no longer required. Other than using up charge cycles, it won't hurt anything. But it also won't accomplish anything useful, either.

Let it die out charge it up to 100% don’t take it off the charger at 100% let it sit for a couple more hours use your device again and repeat process after it chargers up to 100% the next time power off and back on

In theory, 0% is not really 0% just to protect against things like this. However, your advice is still wise.