This isn't an account, but it covers anything an account could.
They update their prices based on how many USD they want for the item and the current BTC/USD ratio. There's even an electronic price tag that does this automatically: http://www.bbc.com/news/technology-26031331.
Online, it's usually handled by some outside payment process company like Coinbase. You set up the website to communicate to Coinbase "I need $x for this guy's order" then they redirect the user to Coinbase's payment page, where Coinbase asks the user for $x worth of BTC at the current price. When Coinbase gets the user's coins, you (the merchant) gets your money. Most merchants never touch the BTC.
In person, without the electronic price tags, it would be impractical (or even impossible) to price items in bitcoin by hand. One way around this would be to price them in USD, then take their total at the register to determine the amount of BTC needed for the purchase. Ie, the customer comes to the register with $x worth of goods, the merchant looks up the price of BTC (which they could do quickly), then asks the customer for $x worth of BTC.
If you mean how do they manage the risks of holding an asset that is volatile, the answer is that they don't hold it. They use Coinbase or similar.