At this very second, cells in your body are replacing themselves. Maybe your muscle cells, maybe your bone cells, or maybe even both. First, how do they do it?
All cells replace themselves by dividing, but the results of this division vary in different kinds of cells. Some cells, such as those of the liver, divide in two to create two liver cells which are just like the parent that divided to produce them.
In other cases, special cells called stem cells divide to replace other cells. That is, the cells produced are not always just like the parent. The basal cells of the skin are stem cells. When a basal cell divides, one of the two daughter cells becomes another basal cell to replace the parent. The other daughter becomes a skin cell. Some stem cells can even make various kinds of cells, depending on what your body needs.
How often do cells divide? Some cells routinely replace themselves, such as those of the inner lining of the small intestine which replace themselves in a week's time or less. Other cells stick around for a long time; pancreatic cells can last a year or longer.
Still other cells vary in the rate they replace themselves. For instance, the rate at which basal cells in your skin replicate depends on the rate at which the outer layer of your skin sheds. If damage to the skin causes it to shed faster, then basal cells respond by dividing more frequently.
While the ways and rates at which cells replace themselves vary from one organ or tissue to another, one thing is constant, our bodies are never constant, but always changing.