"Relieve stress" is an older, more mainstream term. Anything that reduces stress -- medicine, diet, exercise, laughter -- is said to relieve stress.
"Release stress" is more of a self-help, new-age term. It's commonly used by people who practice yoga, meditation, and guided imagery. Most often, it refers to reducing stress through relaxation and, occasionally, homeopathic (herbal) remedies.
Lots of luck,
- For Teachers