I replaced my stock B1250 windscreen with a MRA Vario Touring Screen, but I'm pretty sure this procedure will apply to any aftermarket screen, to all '05+ B650's, '06 B1200's, and '07+ B1250's.
First remove the mirrors. This is necessary because the screws that hold the mirrors on also hold the outer fairing shell to the inner panel. Later you will need to pry the gap between the two open far enough to fascilitate the removal and replacement of the windscreen without risking breaking anything.
Each mirror is attached with two allen screws, capped with little plastic plugs. The plugs pop right off with a small screw driver. In this pic the upper screw has the plug still on, the lower one has been removed:
Now remove the two upper fairing/windscreen screws (yellow arrows) and the two instrument pod screws (blue arrows):
Grasp the instrument pod under the lower edge and pull up gently, until it pops loose. Then turn it upside-down and place it over to the left out of the way:
Now you can access the two lower windscreen screws. The one on the left side:
And the one on the right side (note the rubber grommet circled in red- this is where the white plastic retainer on the back of the instrument pod plugs in):
There are only 4 mounting points. The screen slips out and the new one slips in between the fairing and the inner panel. Shown here are the right side mounting tabs, the left side is identical:
Replace the screws and instrument pod in the reverse order that you removed them, followed by the mirrors and you're all done.
<NOTE> In the final pic, the new windscreen is already installed, and the old screen is just laying on top to show where the mounting holes line up. That's why the mirrors are on there... I was already done with the installation.