There are several methods that expedite the ripening process of an avocado, but placing it in a paper bag along with an apple or banana is the best one.
It will still take a day or two, depending on how unripe your avocado is. The prolific ethylene gas apples and bananas produce help avocados ripen faster.
After 24 hours, check the avocado; if it is not ripe, return it to the bag and give it for another 24 hours. Use the avocado right away once it ripens or store it in the fridge.