You can't adjust it, it is controlled by an idle air control valve ( IAC ), your car may need a new one. http://www.ebay.co.uk/itm/Idle-Air-Control-Valve-VOLVO-240-440-460-480-740-940-S70-V70-0280140516-/350586480732
Your welcome, a temporary fix would be to remove your IAC and soak it in paraffin overnight, the needle inside them that controls the air flow tend to get gummed up sometimes.
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