Radio controlled cars, boats, helicopters and planes all have one thing in common, commands are sent to the receiver using radio waves. Newer RC systems use a "spread spectrum" for broadcasting and receiving signals, and these systems don't use a traditional crystal set. However, older FM-based radio systems do use a matched set of crystals, one in the transmitter and one in the receiver. If two radio systems have the same frequency they can not be used at the same time, one will interfere with the other and cause an unsafe condition. One way to avoid this hazard is for one user to change crystals.
Locate the receiver crystal, it is silver in colour and about the size of a pinky nail. There is no standard location for the crystal but since receivers are often sold without crystals they are always accessable. Remove the receiver crystal by gently grabbing it with your fingers and pulling it straight out of the saddle. If you can't get a good enough grip on the crystal with your fingers then use a pair of pliers. Be careful not to grip the crystal too hard; you could break it. Some manufacturers attach a length of coloured tape with the frequency printed on it to the crystal; if so, grab hold of the tape to remove the crystal.
Remove the transmitter crystal in the same manner as the receiver crystal. Some manufacturers mount the transmitter crystal outside the case, making it easy to change; others mount the crystal inside the case. Internally mounted crystals look just like the receiver crystals, those mounted outside usually have a plastic case attached that has the frequency printed on it. This plastic case is what you pull on to remove the crystal. You may have to search around for an internally mounted crystal as there is no standard location for them to be mounted.
Install the new crystal by pressing it into the saddle that the old crystal was sitting in. It does not matter how the crystal is positioned as long as both prongs are seated in the saddle. In most cases you can not use a transmitter crystal in place of a receiver crystal.
Be sure to range check your radio system after changing crystals to ensure that everything is working correctly.
Don't power on the receiver with no crystal or the wrong crystal installed, this could cause servos to be damaged.