If the serpentine belt breaks on a car the alternator will quit charging and the water pump may not work, causing the car to overheat. Before the belt breaks it is good to inspect it for cracks on the inside where the ribs are located. Also if the belt is worn the topside of the accessory belt will look rough instead of smooth. In the picture you can see the arrow pointing to the belt tensioner. Depending on where this is located will judge how tough of a job it is to do. There is a breaker bar connected to the tensioner in this picture. The breaker bar is a 1/2 inch drive bar. Some tensioners will have a nut to place a wrench on instead of this square hole for a breaker bar.
This is the block that the 1/2 inch breaker bar fit into, if there were a nut instead it would be located to the right of this block. After you have the wrench or bar connected, rotate the tensioner in a direction opposite where the accessory belt is. In this picture the breaker bar would be rotated to the right. When you do this you will see the belt loosen up. Simply slide the belt off the tensioner and take the pressure off the tensioner. You can now remove the belt easily.
Somewhere under your hood a sticker like this showing the belt routing, will be found. To reinstall the accessory belt route it around all the pulleys as shown. Leave the belt off of the tensioner pulley. Now use the breaker bar or wrench again and rotate the same as you did to remove the belt. Carefully slide the belt back over the pulley and slowly let off the wrench. Inspect the belt to verify that it is routed correctly and is not part way off of any of pulleys. Check again after starting the vehicle, if the belt keeps coming off the belt tensioner may be bad and need to be replaced.