This is how I fixed my E90 seat. E70 may be similar. Remove the plastic with the switches on the side. Then un-clip the cover from the lower metal frame. The entire cushion and cover will now come off. The cover is held to the cushion with a few hog ring clips. Replace the cushion and reinstall everything. The cover can easily be re-died if there are areas where the leather color is worn off.

EDIT> I just checked my comfort seats. They are more complicated than my E90. I would recommend you tilt the entire seat assembly back to get a better view of how to dissemble. Remove the 4 bolts that go into the floor. That should allow you to lean the seat back and see what is under it. If you want to unhook the wiring, unhook the battery first to prevent your airbag light from coming on.

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2009 35d, 2006 325i & 330i