You mean change? What make and model?
Raise power limit. Don't expect more than ~1050Mhz though.
The r9 270 has no voltage control by design (like Nvidia GPUs , it uses onsemi voltage controller) and there's a limitation of the physical power connectors (one 6 pin). Some aftermarket cards like the one by ASUS deviate from this and include a rebranded CHiL PWM controller (marked digi+) , you might get 1150Mhz on those.