These days almost all methods available to buy bitcoin also offer the option to sell.
The exception is bitcoin ATMs – some allow you to exchange bitcoin for cash, but not all. Coinatmradar will guide you to bitcoin ATMs in your area.
All exchanges allow you to sell as well as buy. What kind of exchange you choose to sell your bitcoin will depend on what type of owner you are: a small investor, an institutional owner or a trader?
Some platforms like GDAX and Gemini are more targeted at large orders from institutional investors and traders.
Retail customers can sell bitcoin in exchanges such as Coinbase, Kraken, Bitstamp, Poloniex, etc. Each exchange has a different interface, and some offer related services such as secure storage. Some require verified identification for all professions, while others are more relaxed if small amounts are involved.
(Of course, do not forget to declare any profit you make on sale available to your relevant tax!)
You can, if you wish, replace the bitcoin for other cryptoassets rather than cash. Some exchanges such as ShapeShift focus on this service, allowing you to switch between bitcoin and ether, litecoin, xrp, dash and some more.
Another alternative is direct selling. You can register as a distributor on platforms such as LocalBitcoins, BitQuick, Bittylicious and BitBargain, and interested parties will contact you if they like your price. Transactions are usually made through deposits or wires to your bank account, and then you are likely to transfer the agreed bitcoin amount to the specified address.
Alternatively, you can sell directly to friends and family once they have a set bitcoin wallet. Just send the bitcoin, collect the money or charge mobile, and have a festive drink together. (Note: It is usually not a good idea to meet with strangers to exchange cash bitcoin in person.
(Note: Specific businesses mentioned here are not the only available options, and should not be treated as a recommendation).