Bus/train from terminal 5 at Heathrow to terminal 2.

Train is easier, and free.
AFAIK there is no BUS. The train goes direct, under the taxiways. By road, you have to exit the airport, round the edge of the airport and take the exit for Heathrow central (T2/3), a long way.

BTW, I assume you are talking about landside, NOT an airside transfer between flights.

The choice is between the shuttle trains or the London Underground Piccadilly Line. Both are free although for the latter you need your own Oyster card or form of contactless payment to enter, although this is not subsequently charged.