How do I remove plastic-headed wiring staples?
You can use the claw of a hammer, a cat's paw, a small pry bar or a flat blade screwdriver as a prying device. Slip the prying device between the plastic &amp; the cable, or slip it between the wood and plastic and pry. Pry gently if you're between the cable and the staple - you don't want to dig into the cable housing and damage it. You can also twist to gently begin moving it away.

If the wire staple is really tight, you can use vice-grips* to grab the plastic around the nail and pry it out a bit until you get enough room to get under there to pry it out. (Or, just keep bending with the vice-grips until you get the nail out - they're not that long.)

Depending on the amount of effort that goes into removing them, you may be able to reuse the wire staples to resecure the wire in its new location, or you may mangle it far beyond reuse. Have a small bag on hand for replacements. It's easier to return the bag unused, than to be up in the attic when you realize you need a few.

*Using "vice-grip" as a generic term for locking pliers, no specific endorsement of Vice-Grip™ brand locking pliers implied or intended.