Typically, turning to the right (tightening) will straighten the neck, or put it in a back bow, if already straight. Loosening will create a forward bow ( moving strings farther from the neck. De-tune your strings first. Use you string as a straightedge and sight the neck when adjusting. If you want to be cautious, only adjust by a 1/4 turn each time. Make your adjustment, and re-tune the bass. Give it a few minutes and re-check. It is possible to go too far, too fast, and possibly ruin your truss rod and neck. If you get worried about doing it...just take it to a pro. Good luck and take your time!
Spector Club #251; Eden WTDI Bass Preamp Club # 2; The Fretless Club #731; Yamaha BB Club #44