Make sure the VCR is properly switched on (and not just in standby mode - Press the power button on the handset). Then, simply pressing any numbered button should change the VCR to receive the appropriate channel. However, in your case, you don't require any numbered channel. You need to press the 'AV' button, instead of any number. (On a few VCRs, pressing '0' might do the same job).
The only reason I can think of why you might not be able to change the channel is that the VCR could be set to record a programme using the timer. In which case, you might need to clear the timer setting first. Unfortunately, since no two VCRs are alike, I haven't got a clue how to do that with your model. It will just have to be trial and error.
Chris