Remove the 3 screws marked in red before careful removal of the black cable marked with red arrows along the edge of the fan. During re-assembly, remember to re-thread the cable through its hooks as to preventing it getting in the way of the fan or optical drive.

Use a plastic spudger to assist in disconnecting fan's power cable by pushing it in the direction marked by the red arrow. Take care to only apply pressure on the connector itself as to prevent damage to any other component on the motherboard. Light tugging on the cable itself once loosened can help.