There are many yoga stretches you can do at your desk to relieve tension. Try the following 

• While sitting at your desk, inhale as you lift your left shoulder to your ear.  Exhale as you slowly roll your shoulders around and back, dropping away from the ear. Continue these shoulder rolls 3-5 times, alternating left to right. 
• While sitting at your desk, cross your right leg over the left. Place your left hand on the crossed knee. Gently turn your body towards the right, looking behind you. Repeat 3 times on each side. 
• While sitting at your desk, reach your hands toward the ceiling.  Inhale deeply while focusing your eye gaze upwards.  As you exhale relax completely and lower the arms. Repeat 3-5 times.