You can make this change in the ARM profile this is under the RF Radio profile either 11g or 11a .
I recommend you create a new one ARM profile for that particular AP-group so if you have to make the change only do it for that AP-Group / APs tied to that AP-Group
To do it on a certain AP you could use the AP Specific and instead of inheriting the settings you could create a new one
Note: Make sure that you test this (perform a site survey) or use Visual RF on Airwave or if you dont have airwave you could use the standalone version , because increasing the power on could potentially impact wireless performance if the APs hear each other too much