How to Prune alum root 'Isla'?
No pruning required. Remove dead leaves

alum root 'Isla' · Heuchera 'Isla'
Grow in fertile, humus-rich, moist but well-drained soil, in partial or full shade, and mulch every year; the rootstock tends to push upwards, so every few years, in late summer or early autumn, divide and re-plant with only the crown above ground
No pruning required. Remove dead leaves