Hi. If you urgently need to prune now in autumn you will lose all flowers which would have been out early next year. I think best time if you can wait, is just after they have finished flowering. The buds are on the plant for a long time.

Its always best to avoid pruning if you can apart from removing the dead flowers. If it is unavoidable for some reason then mid to late spring is the time - try to restrict the pruning to weak and/or dead-looking branches but again if you need to reduce the size then you can cut back although anything more than superficial cutting may knock back the flowering in subsequent years.

Reducing the size can be done in stages:

Right after blooming, cut 1/4 to 1/3 of the longest branches to 30-50 cm inside of the outline you want it to be, eventually. Repeat every year for 3-4 years.