Edit local.config
Change/add these values:
cor <you.rig.name> <GPU0.core.clock> <GPU1.core.clock> <GPU2.core.clock>...
mem <you.rig.name> <GPU0.memory.clock> <GPU1.memory.clock> <GPU2.memory.clock>...
Example:
cor f3sh85 1350 1325
mem f3sh85 2300 2350
Anything over the base clocks will be overclocking
You can also play with the powertune,
5 is the suggested one