Easy Tube route from Heathrow to Liverpool St, London

Hi Folk

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.

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.

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?


Rgds

Calvin