if it has a breather box, remove the box cover or the whole box depending on the model and make, and pull the old breather out or possibly loosen the retaining clamp and pull it out, replace in reverse order. If it is just an exposed breather, just the clamp.
Locate filter housing around the engine, remove bolts holding in place. This will expose the filter. Simply pull the filter out, replace with new filter then replace housing and bolts.
Despite the fact that it may say to service your breather in your manual, this is typically not done. Picture one of those metal scotch brite pads, this is your breather and you don't generally touch it.