Help in converting

I have a 4 channel transmitter and receiver.how to convert it to
20 channel.

You'll need to add a load more information than that if you want help. Pictures and model numbers would be a start. Do 20 channel transmitters exist that are similar to your 4 channel one?

You haven't even said if it's a radio, infra-red, ultrasonic or something else.

I have 4 channel 6CH remote control UNIT 27Mhz PCB radio module (used in rc cars). I want to make it
20 channel one.

RC radio uses pulse width modulation. The first channel is sent out as a 1 to 2mS pulse, the second after approx 2.5mS and the whole sequence repeats every 20mS. Therefore, you can fit a maximum of 8 channels in the 20mS time frame. You don't say what country you are in but some countries don't allow 27MHz anymore due to ground models causing airborne models to crash.

Mike.