The indicator lenses just slide along a channel, and are secured in place by the long arm which clicks into place. When you get the new part, the arm will be obvious, then just find the protruding end of it in the engine bay, push it in to release it, and the old unit will slide out. On my 500, iirc, I have to lift up the washer bottle on the n/s for the headlamps to gain access to the arm. I've never changed a headlamp or lens, but if it is like the 124, which it probably will be, it should be simple and intuitive, especially with the indicator lens already removed.
Hurry up with these pics, we're not going to notice (or care about) the odd nick here and there!!