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First of all we take out the lens we want to change pulling the upper part of the frame to unlock it from the two upper joins. When this will be done the lens will come out easily from the sides and from the nose part.
Now we can take the lens that we want to mount and we insert in in the slot in the frame strarting from one of the two sides: it's important that the lens is inserted deep in the slot. When one side is done we just have to do the same thing on the other.
Only when we have inserted the lens in the sided we can start with the bottom and the upper part. First we insert the bottom part of the lens in the slot of the frame, again, deep in it, pushing with the finger until locked, then we can lock the upper part, paying attention not to push too much the lens on the joins not to deform them.
Once this is done, if the nose part would result "weak"you can push with a certain decision the nose part without unlocking everything else.