There is a slide lock at the bottom on the end of the PCIe slot that is closer to the front of the case than the back, that needs to be pulled towards the front of the case to unlock the the card AND there is a screw at the other end of the card that attaches it to the case. Both need to be done and then you need to gently grab the card by the metal bracket on the one end and the PCB or fan housing on the other and gently rock it from end to end of the slot while pulling upwards. It will come out. Be sure not to accidentally slide the bottom lock back in place or you can break it while removing the card.
Some motherboards don't actually have a slide lock at the end of the PCI slot, but instead only have a "lever" that you must pull...