A couple of things can cause high idle speed.

Sticking throttle linkage. Make sure it's moving freely.

Idle spped is controlled by the indle amp. It it's bad ( and they often are, idle speed will go up )

Over voltage protection relay. There's a fuse inside of it. If the fuse blows you have to get a new relay - it's red in colour.

Idle speed actuator. It can jam up and stick open.