What's the easiest way to change a alternator belt

The easiest way is to have a mechanic do it. There is only one way to install a new belt, do it yourself or hire it out. Your choice.

I'm going to do it dont have enough coin

You have one belt, called a serpentine belt, as it drive all the accessories (water pump, power steering, alternator, and AC). If the old one shredded, you may need to investigate if one of the pulleys has seized up, by spinning them all by hand. A bad pulley can be hard to detect tho, as it might not be apparent. These belts are installed by slacking off the tensioner pulley and then sliding the belt on according to its routing diagram. In addition to the pulleys that drive the accessories, there are two other pulleys to be aware of, the tensioner and the idler. Those more often are the culprits for belts slipping or falling off. The engine overheating was likely caused by the loss of the belt, as the water pump would no longer work without it. Do not drive vehicle until belt is replaced as it can quickly overheat again. If you prefer to have a mechanic do the work, save yourself the cost of a tow by having a mobile mechanic come to you.