Is there a way to rename relationship class labels (forward and Backward labels) after I've created them ? or I have to delete them and create them again ?
Base on the Arcgis Help, you cannot change relationship class properties that were assign during the creation of relationship class. Link: http://resources.arcgis.com/en/help/main/10.1/index.html#/Viewing_and_modifying_relationship_properties/004t0000000q000000/
So, You have to recreate you relationship classes!
Use ArcCatalog to rename relationship classes: