Generally, it is best practice to relieve the belt tension before removing/replacing. Typically, belts are tensioned by a spring load tension pully or some accessory that slides on a bracket.
Good Lord man. Don't be daft and get hurt with poor mechanic skills.
FWIW, I've had very good experience using Gates belts and drive products.
Another way to tell if the belts are worn. Place a straight edge across the pulley flanges, if the top of the belt is below the flange edge, then belt is worn needing replacement.