Liemack fridge question

Hi all, Just wondering if anyone knows how to recalibrate the voltage reading on a Liemack fridge, voltage display is reading 13.4v actual voltage at the batteries is 12.4v. ( I have searched the archives without lany uck)

I had this problem years ago when Liemack were still operating and was told how to do it by their tech. I think I had to hold down 2 buttons at once and then I could alter the reading with the up and down buttons.

I have tried a few different combinations but can't get it to do it, any help greatly appreciated.

Thanks Neil.