Without adding a manual boost controller to adjust boost, a manual tweaking that can be performed to accomplish manual boost adjustments is fine tuning the wastegate bolt. One of the simplest ways to accomplish this is to adjust the bolt (inward) on the end of the wastegate. However, this will only work on external wastegate models. Care should be taken when performing this adjustment because the wastegate can be bound, resulting in an undesirable over-boost condition. The safest and most reliable way to adjust boost is adding an MBC (manual boost controller), and adjusting boost from the knob/actuator built into the MBC unit.
For more info, see Car Tuning: Turbo Tips