Calvin Chan wrote:

My family is travelling to London in a week's time. We are carrying
quite a bit of lugguage, and landing at Heathrow. We are planning to
take the Tube from Heathrow to Liverpool St.

The tube isn't very good at handling people with "quite a bit of
luggage." Travellers with one or two bags are much less of a problem.

I was told that London Underground network is very complicated, and
the changing of lines may involve lengthy walk and lots of stairs to
climb up and down.

Very complicated? Not really. Stairs and lengthy walks when changing
lines? Quite possible, and sometimes, not a problem at all.

Anyone out there can tell me how I go in changing lines in a easier
way not to involve a lot of walk and stairs?

How about not using the tube at all?

Take the Heathrow Connect train from Heathrow to Paddington Station,
then pay for a taxi to take you to Liverpool Street Station. Costs
more, yes, but it's a far more practical solution for travellers with
"quite a bit of luggage."

If you absolutely must take the tube, then you can change at Paddington
for a Circle or Hammersmith & City Line tube train to Liverpool Street.
You'll have to negotiate stairs there, though.
--
dgs