Turbo boost maximum level is maintained by the "wastegate". Turbos have either an internal or external wastegate.

A wastegate is designed to "redirect" air around the exhaust turbine, so the turbo maintains its speed once it hits the boost limit specified by the factory, instead of rising with engine revs until redline.

All boost control mechanisms rely on "tricking" the pressure signal sent to the wastegate actuator. By "bleeding off" pressure signal to the actuator, the boost produced by the turbo will be raised as the exhaust gas is allowed to spin the exhaust turbine faster than normal.

People who upgrade their exhaust, also see rises in boost, sometimes as much as 3psi difference.

How to rise your boost, buy a boost controller or boost bleed valve. The boost controller will generally give you a more consistant rise in boost, as it should be able to monitor what it is doing.

People also put a spring onto the wastegate actuator to make it open more slowly.

The actuators rate of opening and the wastegate's port size will determine the boost control characteristics.

There you go.
Chris

ps. Sorry there is no screwdriver adjustment for boost control.