Remove two or three of the tallest branches, taking care to cut back to a side branch which is heading in the desired direction. Cut close to a side branch, as stubs tend to die back to the side branch and leave dead wood which may become infected later. If you remove a lot of the branches, it is a good idea to mulch around the plant to make up for the reduced shade on the roots. The new shoots, which will appear when the weather warms up, may come from the stubs and from the base of the plant. Use clean cutters, and sterilize them as the work progresses, particularly after a cut in infected wood.
Denatured alcohol is an effective sterilizing agent, as is rubbing alcohol or household bleach.
Next year take out two or three more branches using the same process, spreading the pruning over a three year cycle. This approach will result in the plant sending out new growth near the base, and lets you manage the shaping of the plant to achieve a nicely shaped bush. It also minimizes the shock to the plant. However, they are subject to a number of problems caused by infectious agents, insect pests, weather extremes and nutrition deficiencies . The problems page describes some common problems.