Not all hydrangeas change colors, but many pink-to-blue varieties can be changed by controlling soil pH. In those cultivars, like LA Dreamin' or Twist n Shout hydrangeas, the level of soil acidity will affect the color of the flowers. These flowers become more blue in acidic soils, and lean to lilac or pink colors in less-acidic soils.
To make your hydrangea flowers more pink, you'll need to raise the soil pH (for those of us that don't remember chemistry, that means making the soil more alkaline and less acidic). Try adding wood ash or commercially-available lime to the soil, but be careful not to overapply. Follow the instructions on whatever products you decide to use.
To make your hydrangeas more blue, you'll need to lower the soil pH (making the soil more acidic). However, low pH won't make your hydrangeas blue on its own. Aluminum is the key to turning hydrangeas from pink to blue, and an acidifier without aluminum in it may not achieve the same effect. Look for amendments that include aluminum, and check your local garden center or hardware store for specifically-formulated additives for hydrangeas.