The most popular way is with something like an Apexi Power FC, which is a full replacement ECU that needs to be set up on a dyno. There are others that do the same sort of thing, like the HKS F-Con 5, Motec and Links ECU's. Then there are things like the Unichip, GReddy E-manage and HKS-F-CON S, which are piggy back computers that can adjust a signal on it's way to the ECU, thus changing the parameters.
All of the above are quite serious tuning options to go into. If you're at an early stage of tuning, you may well prefer to just swap out the ECU for one that's being re-mapped by a tuner. This will have been done on a donor car already and will cater for certain mods, usually adjusting fuel, ignition and boost settings so the car doesn't det.
Give Abbey Motorsport a call, as they can help with this route (if that's your choice) before you go all out for a standalone management system to handle the serious mods later on. Mark or Tony @ Abbey are on 01883 732331.