Are you trying to replace the BMW roundel or the 'X5' badge itself? I've de-badged my truck before but never took the actual roundel off. Regardless, from what I know you should heat up the badge first (I just used a hair dryer). This will warm up the adhesive and if you go at it with floss or I used a rubber spatula type tool, it should wobble off.
Hopefully this helps..

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11' BMW X5 M
01' BMW M Coupe

-Elliot