Angel trumpet, also known as datura and brugmansia, is an exotic plant that only blooms at night--with the coming of dawn the blooms close and are gone. The scent of the angle trumpet flowers is almost over-poweringly sweet and fragrant. If you are happy with the shape of your plant and with the number of blooms it is producing, then you do not need to prune it. The two primary reasons for pruning a trumpet plant are to re-shape it and/or to encourage new growth and additional blooms.
Decide on the shape that you want your trumpet plant. If you wish to encourage it to grow taller wait until there is a Y in the primary trunk before pruning.
Use your shears to remove old growth (not this year's growth) from the top of the plant, if you wish to encourage the plant to grow taller. Cut old wood back to the trunk, but do not cut the trunk itself. New branches will grow around the area of your cuts.
Cut old-wood lateral (side) branches back to the trunk--being careful not to cut the trunk itself--to encourage the plant to grow wider and bushier. New branches will grow from the area of your cuts.
Cut new growth only to shape the plant and cut new growth sparingly, as this is where blooms will form.