How to adjust the water level in cistern to stop overflow (SIAMP OPTIMA 49)?

I have a toilet with a Siamp Optima 40 dual flush mechanism. When I flush, the water in the cistern quickly fills to a level roughly half way up the overflow. But if you leave it for a while, it slowly keeps filling it until the water gets to the top of the overflow and starts to trickle into the bowl. How do I make an adjustment to make sure that the water level is never filled beyond the overflow? I've printed some stuff off the Siamp website but don't really understand it. Grateful for any help.