General relaxation:
* Détendre (verb) - This is the most common word and means to relax or loosen something, including tensions.
* Relâcher (verb) - Similar to détendre, but can also mean to release something.
* Déstresser (verb) - Specifically means to relieve stress.
Specific to physical tension:
* Détendre les muscles (verb phrase) - This means to relax the muscles.
* Relâcher la pression (verb phrase) - This means to release pressure.
In a figurative sense:
* Apaiser les tensions (verb phrase) - This means to soothe tensions.
* Désamorcer les tensions (verb phrase) - This means to defuse tensions.
The best word will depend on the specific situation and the type of tension you want to describe.