You must prune a birch tree during the fall when the tree is dormant. Remove any dead branches and debris. Bottom branches should be trimmed so lawn equipment can easily get underneath and to raise the living canopy. Angle your cutting device away from tree and make a clean cut.
An important factor to remember about Birch trees is their wide and shallow root system. Be aware that you must not trim the tree so that it is off balance or it may literally fall over. Trim most branches back to the ground or the trunk if possible to prevent multi-shooting upon regrowth.