Is your travel on one ticket? ie: you can check your luggage all the way through?
It's fairly straightforward - follow the purple flight connections signs and you will end on an air-side bus transfer.
You will need to go through security again.
Otherwise you'll need to travel on the free Heathrow Express from T5 to T2/3 to check-in your luggage, go through security etc.
It's all here:
https://www.heathrow.com/flight-connections