You recalibrate it before you turn it on.

On about 90% of escs i have known its goes like this.

Turn it on, but hold the power or program button depresssed, when it blinks, release the button, pull the throttle trig to full throttle and hold (set throttle endpoint)

It should blink or beep, then push the trig to full brake and hold (brake endpoint)

Then it blinks or beeps again and let the trig return to neutral (set neutral center point)

While your esc might be different, this is probably the sequence you should follow.