Turn the bottle upside down, then hold under hot (as hot as you can handle) running water. The water needs to hit the cap where it screws on to the bottle. Keep turning the bottle for a while, while it's under the hot running water. The hot water will make the cap expand. Dry off the bottle and cap and open immediately before it cools down. Hold it in a dishtowel while opening in case it slips from your fingers. You can do this with any bottle; e-liquid or otherwise. Sometimes the contents of the bottle gunk up the lid and the hot water will loosen that gunk.