Not sure what car you are talking about although I assumed it is something
like 10 years old. For a start if your battery is 6 years old it is probably
overdue for replacement, especially going into winter.

On the oldtimers, the standard regulators are stamped "14v".
Unless you live in a severely cold climate (well below zero °F
and the car stays outdoors all night) 13.9 should be sufficient.
The difference in charging current between 13.9 and 14.2 volts
against a battery voltage of 12.6 is going to be pretty small
and probably will mostly evaporate water out of the battery rather
than increasing its charge.

If I were you I would put in a new battery unless you have an adequate
support system (battery charger, another car and jumper cables, etc)
They just don't cost that much.
--
George Downs Bartlesville, Heart of the USA!