When multiple microphone sets are used simultaneously or if there is any interference in your location, you need to change the frequency.
Make sure both the mic and receiver are on.
Unscrew the battery cover of the mic and shortly press the set button beside the battery compartment. One channel / 0.2MHz increment per press.
The frequency on the receiver will be the same as the frequency on the receiver when the mics pair with the receiver.
If you use more than 2 mic sets simultaneously, you will need to have at least 2MHz frequency separation between every two mics.
You can refer to the video from 3:56 to 4:26.