In an extreme case...say the beer is overcarb'd and you have company coming over tomorrow night...
- Shut off gas.
- Release pressure from keg and remove lid.
- Dip a sanitize paddle into the keg and (without aerating) gently move the paddle back and forth in the beer.
- The beer will suds up and foam.
- Leave it sit for 30 minutes and repeat several times.
You are basically knocking the CO2 out of solution.
Now you can re-lid, purge the O2 and set to a low PSI and test.
If it's a bit flatter than you like, set the PSI to 20 and taste test every 2-3 hours until it's right and drop the pressure.
I've had to do this before, but only when it was critical to get a keg ready for a party. Doing it wrong and you risk oxygenating your beer which is not good.