Thank you for your answer, but I still cannot find the hole or the key at the boiler underneath as you mentioned.
at the boiler underneath there are two big pipe (around 1" or over in diameter) in parallel with the boiler panel face width, there is a plastic ring in black color just next to one of the big pipe, a screw appeared at the end of the ring. I try to turn this ring with my fingers but it seems it is firm.
If possible, could you send me a drawing or a photo to show the right place? I have doubt that I found the wrong ring.
The Boiler is:
WORCESTER (Bosch Group).
Greenstar Si Compact.
Wall Hung RSF Gas Fired Condensing Combination Boiler.
Model :WORCESTER 25Si Compact.
Purchased in year 2014.
Thank you very much your help again.