New Hampshire requires UM equal to your liability limits, treats underinsured drivers as uninsured to the extent of the shortfall, and leaves stacking to your policy
New Hampshire's rules are in RSA 264:15 (UM required and the release warning) and 259:117 (what counts as uninsured). Liability limits are in 259:61, the fault rule in 507:7-d and the deadline in 508:4.
Read how stacked and single-limit coverage differ in New Hampshire, check what the statutes say and leave to your policy, or see how New Hampshire UM covers underinsured drivers.