Which engine brand are you referring to?
Most turbine brands use their propriatary ECU, so the procedure varies from brand to brand. You need to be more specific.

In all of the turbines with which I am experienced, when you reduce the top end RPM, that is the only function effected. No other parameter will be altered.

Reading your post indicates you already have an engine, so why don't you read the manual for the procedure for you engine?