Well Dad used to have two ways of doing it. First was to spray it with glass cleaner and carefully peal it up using ah razor blade. The second ,and much more comical  ,was to cut the glass out around the sticker ,and sand the edges. Now you have a mobile sticker that can be put in any car...We never paid for beach passes...

Try "Goo B Gone" and a razor blade. Start at one corner and be careful.
 Good luck !

I have done many of those.  The stickers in Texas has cut lines all through them so they come off in little pieces, unless you know this trick.

Get some clear packing tape like they use to seal up boxes.  Apply tape to cover the entire surface of the back of the sticker.  Be careful to not get bubbles or creases in the tape.  Rub it hard to be sure it sticks well.  With a single edged razor blade, trim off any tape that extends past the perimeter of the sticker.  Use windex and the single edged razor blade to start working from one corner.  Keep the blade and the area where the sticker just released covered with windex.  Grab one corner and keep working the seal with the blade, holding the sticker tightly and at about 60 degree angle to the glass, keeping it wet with windex.  You should be able to remove it intact, then let the windex dry and stick it to the new windshield.  The ones in these parts will stick really well the second time around.  If it doesn't stick well, cover it with some more clear tape, but this time carefully trim the tape about 1/4" out from the perimeter so the tape adheres to the windshield.

Smother is the new "S'more", and I'm using it!