How can diuretics be used to decrease blood pressure?

Diuretics can reduce the amount of Na+ that is reabsorbed by the nephron (leads to excretion of more salt and water in the urine)
The reabsorption of Na+ from the kidney tubule back into the capillaries lowers the water potential of the medulla. This drives the reabsorption of water from the kidney tubule via osmosis. If the amount of sodium reabsorbed is decreased, then the amount of water reabsorbed will also decrease. This means more water will be removed from the body in the urine, lowering the water content of the blood. This in turn will reduce blood volume and therefore blood pressure.