You can fire a Crew Chief or Driver during the recruiting period. If they have any seasons remaining on their contract you will have to pay a severance fee and lose some reputation points.
You may also fire your second Driver at any time, but you will have to pay a severance fee and lose reputation points if it is not during the recruiting period.
To fire a crew member, go to the Crew Management page and click on the Fire Driver/Crew Chief button. If there is no Fire button, then that crew member cannot be fired at that moment.
Please note that towards the end of the season, a crew member may show 0 seasons remaining, but if you try to fire them before the recruiting phase, you will still end up paying severance and losing reputation.