How do i reset my dryfus combi boiler timer?

I do not have an instruction manual for my timer.
It is currently showing a day ahead. i.e. Today is Wednesday, the timer is showing Thursday. I cannot get the timer to go back one day. The programmes are also not what I would like. The Vaillant combi boiler is operating ok. for both hot water and heating, but the timer is turning the heating on and off at times that I do not want. The timer has six settings. The buttons on the timer are 1.Select i.e, on, off, auto, allday. 2. Programme. 3. Next On/Off. 4. +, -,
5. Day/Hol 6. Copy . There is also an RS slot which will reset to factory settings, to the right of the other buttons.
Could you please email me with instructions that will enable me to reset this timer. Thank you.