I can't imagine that it is much different from the M103.
You need to loosen the large allen bolt (6 or 8 mm key needed) that is located under the tensioner, then back off the tensioner adjustment. That will allow the idler to move and the belt to be removed.
When you put it on, you push the indicator to the end of the scale and tighten the adjuster until the indicator moves to the mark.
There are a lot of good posts on this - try a search.
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