Before proceeding to making any adjustments, make sure your bobbin tension problems are not related to other issues, such as dirt, lint or small pieces of thread inside your sewing machine or the bobbin case.
To make the adjustments, get a tiny screw driver, turn the screw to the right for tighter tension or to the left to loosen bobbin tension.
To test your settings for the new tension adjustments, insert the winded bobbin into the case, hold the tail end of the thread with one hand, place your other hand under the bobbin to catch it and gently jiggle the thread.