Using a spudger, unplug the cable that supplies the motherboard with power. There's a clip to release the locking mechanism; make sure to press and release the clip before pulling up on the connector.

Then, using the same spudger, unplug the HDD power cable.

If you're not used to working with data plugs in tight spaces it could be a good idea to remove the HDD first. Consult the HDD removal guide for instructions.