I am assuming you want to use only the 12V output, this is likely to not work or not work well for numerous reasons.
It is also assumed you want to use it for testing only.
You will need to know what you are doing to attempt this.
Also as Ubergeek' mentions high voltage troubleshooting likely. (PC power supplies have a voltage doubler on the input for 110Vac operation & this generates nom. 350V DC !!!≥ )
The 12V is not the main regulated output for the power supply !
The feedback of output voltage is on the 5V output.
To increase Vout you would have to change the Vout sense resistor (or resistor divider) for the regulated output 5V(/3.3V) by 17% (12 --> 14V =17%) to give 5.75V , this may encroach on the OVP (Over Voltage Protect).
(some PC switchers have a pot where the output can be trimmed, so you might be able to get some increase in Vout here)
Modern PC switchers usually have close coupled output inductors (all on same core) this improves regulation of outputs & might enable some limited success for test use only.
You need to first identify the Vout feeback isolation transformer or optocoupler & then draw out the circuit for the Vout sense section & identify the Vouit sense divider. Then calculate the change required to the Resistor divider.
As with all power supplies you have to derate the output current rating by the % increase in output volts.
since most of them has the tl494 built into them
There is a trend these days to use current mode forward converter & many PC supplies have UC384x controllers.
A more sensible approach would be to place a DC-DC stepup converter on the 5V output to generate 14V.
TG