Hi jamieshevonne,
Don't worry, it is highly unlikely you would kill yourself, but too much vaped nic will give a headache sometimes. Unfortunately, since these are prefilled carts, and I assume you don't have a lot of DIY gear, like syringes or pipets, the only thing you can do, at least initially is vape from them less often. Of course, making sure you know the strength of carts before buying them is important, and I'm sure you will do this in the future, but to not waste these, there are a couple options.
1. Go to a drugstore and
buy some glycerin. Actually I get mine from Whole Foods, Heritage brand, but CVS or Walgreens glycerin will be ok too. Make a liquid which is 20:80 distilled water:glycerin. Mix thoroughly, then add drops to your cart as you vape on it. It will lower the flavor, although menthol is pretty pervasive, and it will dilute the nic.
2. Do the same thing with a e-
juice you
buy online. This will also get you into filling your own carts, which is MUCH cheaper. Make the liquid maybe 12 mg, and add drops to the cart occasionally as you vape. Eventually the liquid will be converted over to 12 mg entirely.
3. Buy a liquid at 18 mg, and drip directly 4 drops onto your atty, and put an empty cart on and vape.
OTOH if these are not carts (cartirdges) for a 3-piece, but instead cartos (cartomizers) for a 2-piece, then you can do this by puffing a while without inhaling on the 24 mg prefilled liquid, then opening up the carto and dripping some of either the glycerin/H2O liquid or a lower nic liquid. Both carts and cartos can be refilled...although some companies don't want you to know that, so you will buy more and more and more.
But this might be the catalyst to get some liquid and maybe even start considering DIY, which is by far the cheapest and most controllable way to vape.