How do I alter the time on my central heating controller?

I have just bought a house which has a Randall 102 timer already installed. I do not have the users manual for it. It is quite old but looks the same as those currently for sale. I want to alter the blue and red timing tabs, but they are stiff and I do not want to break them. I presume that they are meant to push away from the centre of the timing clock. Can any body confirm or supply alternative, please?