It can be pruned at any time of year and to make a shrub bushy prune the middle to ends of a stem to make that stem produce more foliage further down its shaft/length.

If yours are standards and you want to keep them that way, don't prune too hard at the top all at once. If they're not grafted onto something else, that might force new growth down the bare stem, which you don't want. If they are grafted (look for a lumpy bit at the top of the stem) then don't cut beyond that.