The way you adjust the way your camera metering light is usually done through a button on DSLRs or through a menu system. The button, or icon to activate light metering adjustments is usually depicted by the symbol at the left. Once pressed, the three (or four) light metering options are selected by using a dial or buttons to scroll through the options. (refer to your manual).
The three typical light metering modes are illustrated above; they are EVALUATIVE, PARTIAL, and SPOT. Some camera also include a CENTER-WEIGHTED AVERAGE mode. For more on light-metering differences, refer to this article: Light Metering
