If your kegged beer becomes overcarbonated, it is possible to decorbonate the beer.

You should periodally pull the pressure relief valve on the keg, depressurizing the head space in iside of the keg. This will cause the Co2 to be released and it will come out of the brew and repressurize the head space inside of the keg, you can repeat this process as many times as needed.

In extreme case of overcarbonation, you may find it advantageous to leave the keg open to the atmosphere by attaching a gas disconnect to the tank post marked "in", but I would only do this in an extreme case only.