Replacing the side grilles on your BMW E46 M3 is relatively ‘tricky’ but not difficult, you just have to know how to do it.
You’ll need two small flathead screwdrivers. And I mean small, like almost the size of the ones you use to tighten your glasses.
You should see two tabs along the bottom of your gill. You’ll need to squeeze under the plastic and pop those two tabs out. It might be really difficult to get under that bottom plastic piece to reach the tabs, so I used a razor utility knife. If you use that, then you have to be very careful not to scratch the paint (or the clear bra in my case).
Once you pop those two tabs up, then you can slide the bottom out first and then the top. To put replacement gills in, just slide the top in first and then the bottom until you hear/feel the tabs click into place. Let me know if you have any questions.