Short answer is now, with a saw!!
Now it's finished flowering, it's the right time. Just make it into a tidy shape, with no stray stumpy ugly bits sticking up or out to spoil its appearance. It's make plenty of new shoots and should flower again next year. Rduced the one in my Dad's garden to four feet from fifteen or more last year and it flowered this spring.
oh, and the wood is brilliant yellow inside
Hope that helps