The strings of cellos need to be replaced every year or when they start to wear down. Strings can last longer than a year, but if you play regularly, they do need to be changed once a year. Violins and Violas require more frequent string changes, and this replacement tutorial can also be used to remove and install strings on those instruments.
Step 1: Loosen Pegs
Loosen the lower pegs by turning them clockwise or loosen the higher pegs by turning them counter-clockwise. This should cause the string to relax.