You can try to open them and refill it, but as was pointed out earlier it's really not the best idea as they aren't designed with refilling in mind and you'll probably destroy it in the process of trying to open it.
Do a 90/10 distilled / antifreeze mix. That's basically what Corsair puts in there.
Get a large bowl and a power supply you can use to power the pump. Cut the line. Use the bowl as a reservoir to work all the air out of the running pump. Re-connect your cut hose with a straight-thru hose barb connector.
These things aren't meant to be taken apart, but they also aren't rocket science.