errr.... easier to answer the question with pictures than with words......
the first pic shows the link in its open position.......... u can see there is a U shaped part in the last centre link.....

ok.....in order to make it shorter, u need to close the link........ first u lift the bracelet up and rotate the last link around the spring bar that attaches the bracelet to the clasp.....
see the spring bar in the picture below...?
the U shaped depression will fit perfectly on the spring bar and it will then look as below
to make the bracelet longer, reverse the procedures.......
very simple huh.....