If your lens is missing the lens release button or tab, such as in the image below:
Use a small pointed object (such as a screwdriver, paper clip, etc) and insert the tip into the exposed hole
Using the object, pull the lens locking pin down, this will release the lens on the adapter, which can now be rotated and removed (it usually helps to have a second pair of hands to help remove the lens as the pin is pulled back).
Hope this helps, if there are any problems or complications please contact the Fotodiox Tech Support team for further help or information on how to remove your lens from the adapter. Happy shooting!