You can find the user manual here that will explain pretty much everything you need to know:
Section 1.2.3 says the following:
Each transmitter has an individually assigned, unique ID code. In order to start operation, the receiver must be linked with the ID code of the transmitter with which it is being paired.
6
Once the link is made, the ID code is stored in the receiver and no further linking is necessary
unless the receiver is to be used with another transmitter. When you purchase another R8EH, this procedure is necessary; otherwise the receiver will not work. 1) Place the transmitter and the receiver close to each other within 1 meter. 2) Connect CH3(R8EH) to ESC. 3) Press and hold ID SET switch of the receiver one second, now the indicator LED will
starting blinking. It will automatically find the nearest transmitter to bind. 4) Test with servo to make sure the binding is finished.