The tensioner presses against a pivoting curved rail, which presses against the chain. The rail has a removeable plastic cover that wears as the chain rubs against it. I believe that you can replace it without removing the rail by pushing it toward the bottom of the rail. If it won't come out that way, it is fairly simple to remove the rail at its pivot using a bolt and some washers as a puller.
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe