How to Prune underground copiapoa?
No pruning required

underground copiapoa · Copiapoa hypogaea
In milder areas, may be grown outdoors in gritty, poor soil in full sun. Protect from excess winter wet. In colder areas, grow under glass in a mix of 3 parts standard cactus compost and one part perlite. This is a desert plant, native to one of the driest regions of the world; water moderately from spring to early autumn and keep dry in winter. See houseplant cacti and succulent cultivation for more details
No pruning required