How to adjust idle speed?

No adjustment magically adjusted itself that high. Look for a vacuum leak. First inspect the rubber hoses for signs of cracks or splitting. Next, with the engine still cold, use a spray bottle to spray water over the engine. Look for gaskets or hoses where the water gets sucked in and the engine momentarily slows down.

Yup, absolutely. Your car uses a mass air flow sensor in the fresh air intake tube. That tells the Engine Computer how much air, by weight, is going into the engine, then the computer commands the corresponding amount of fuel to go with it. Any air that sneaks in through a vacuum leak or an air leak in that tube after the sensor will not get included in that calculation so you won't be getting enough fuel. There's your low power.

A vacuum leak will raise engine speed but there will not be an increase in power at the same time.

There should be a vacuum hose routing diagram under the hood to show where all the hoses go. The fact that someone plugged them previously suggests there was some other problem they didn't know how to diagnose, or some other component broke or stopped working and they just tried to bypass it. That means there will likely still be a problem once all the hoses are reconnected correctly.