This post assumes you are on the main departure floor in T5C and you wish to go either the main building or to T5B via the passageway.
First you need the right lift, it's between gates C53 and C54, between World Duty Free and the BA customer service desk.
And this shows the lift area, with World Duty Free on the left.
Get in the lift and select -2.
This will bring you down to the transit offloading platform as shown here (repeated photo).
The automated double swing doors on the left open the route to the passageway (it's not the red door). See previous posts for the details of T5B and T5A areas.