On a turbocharged car, the boost control is managed by the actuator on the wastegate which opens and diverts pressure away from the turbo once the inlet pressure reaches a certain limit.
If this this pressure signal from the inlet to the actuator is intercepted, it's possible to control the inlet or boost pressure that the actuator will open at. You can use an electronic boost controller to do this, but the simplest and most cost effective way to do it is with a manual boost controller.

These manual boost controller valves sit inbetween the inlet pressure feed and the actuator pressure intake and are adjusted by screwing the back in and out. Fitting is very simple and instructions are provided

Please note that although the priciple is very similar, these are not 'bleed valves' like the cheap manual boost controllers sold on eBay which are prone to leaking and boost spikes.