Liquid nitrogen changes to a gas when it gets warm enough.
The temperature varies only slightly with pressure; at normal atmospheric pressure it is 77K, or in degrees Celsius -196.
Nitrogen only remains a liquid at very low temperatures (-196oc) above this temperature the liquid will return to a gas, so warm it us is the answer, although because of the extreme cold temperatures...
They are the same element as the name suggests, but due to their different states i.e. nitrogen is a gas which makes up 70% of our atmosphere and liquid nitrogen is nitrogen cooled down to -196...