How can I unset a CUDA event?

I have a processing loop on the host, where I record an event in a GPU stream. Then another stream waits for that event (waits for event's state "set" or "true"). Will this function (cudaStreamWaitEvent) unset this event (so, switching it to "unset" or "false")? If not, what CUDA function I can use to unset this event?

This sounds very much like an XY question. You might be better off describing at a higher level what it is you are trying to accomplish, or what problem you are facing or think you are facing.

cudaStreamWaitEvent does not "unset" an event.

When the event is encountered in the stream, then cudaStreamWaitEvent will unblock, and any subsequent calls to cudaStreamWaitEvent on the same event will immediately unblock (assuming no cudaEventRecord has again been issued for that event). This behavior is easy to prove with a trivial code sample.

The function that "unsets" a cudaEvent is cudaEventRecord(). Any cudaStreamWaitEvent calls issued after that event gets recorded will wait again, until it is encountered again.

You may want to read the runtime API documentation for cudaEventRecord and cudaStreamWaitEvent. Note the following excerpts:

cudaEventRecord:

If cudaEventRecord() has previously been called on event, then this call will overwrite any existing state in event. Any subsequent calls which examine the status of event will only examine the completion of this most recent call to cudaEventRecord().

cudaStreamWaitEvent:

The stream stream will wait only for the completion of the most recent host call to cudaEventRecord() on event.