You go and find the manual. Then you read it.
It took me about 60 seconds using Google. I suspect you may have a paper copy somewhere.
I found this:
https://www.worcester-bosch.co.uk/support/document/download/8716104440
Check page 3
And the user manual:
https://www.worcester-bosch.co.uk/support/document/download/release/8716101859/12822
Mentions reset several times.
XXX, February 2016