If you have an old Mahonia that has outgrown it’s space, use the renovation pruning method. It can be pruned by removing the top third & you’ll remember why you planted it in December when the scent of the lemon flowers waft across your garden!.
If its so huge that you’re wondering why you planted it in the first place or thinking about getting someone to take it out, it’s worth the risk of taking it down to about 3ft. It may not survive but if it does it’ll reward you with architectural evergreen foliage, long clusters of scented yellow flowers & you won’t be left with a hole to dig & a space to fill.
In either case it should be done before the end of April remembering that it will sprout from wherever you’ve cut it & produce the wonderfully scented lemon flowers in December. Well worth a few hours work.