Please when do I need to change my clutch pressure plate and clutch disc

When a problem occurs. Or anytime you feel it would be advantageous to avoid an unexpected disablement. Usually the clutch disc wears out and it shows by slipping in higher gears under acceleration. This indicates time to get new parts. Pressure plate can fail by causing hard shifting/no shifting allowing only when car is off. If clutch has 100K miles that could be a good time to change although it could last to over 150K or more.

Shifting is the process/motion of changing from 1 gear selection to another with the shift handle/stick.