There are three easy ways to fix your candle. Firstly, if the tunnel isn't too deep, you can simply burn your candle for 3-4 hours to create a full wax pool! This should work in a larger candle jar or 3-Wick candle as they have longer burn times.
For mild tunneling where you can still see the wick, you could use a hairdryer. The heat from your hair accessory will melt the candle wax and make it much more even. Once done, follow the steps for lighting a candle for the first time, and melt the first layer of wax completely. Ideally this should stop any occurrence of wax tunnels again on your scent!
With extreme tunneling - where you can barely reach your wick with a match - you need aluminum foil. Before using this technique, trim your wick to 1/2cm, then wrap your candle with the foil so it completely covers the outside. This should create a “dome” above the wax. Leave a hole roughly 2.5cm wide in the top, then light the candle. The heat from the candle should be reflected back onto itself, and after 1-3 hours of burning, the wax should have evened itself out!