At the top of the s7 manager window there should be a menu called view, and you can convert the langue to LAD or FBD.
However, a lot of times STL is unable to be converted. Depends how complex the network is, and also if they included all of the "NOP" instructions.
If you have trouble with some part of it, you can probably post it here and people will help you figure it out.
edit**
Not the manager window, the programming window that the logic appears in, when you are viewing a block