The short answer
Use a licensed bureau de change with an official sign and a posted rate board, not a hotel desk, a shopkeeper or someone on the street. There are clusters of them around Jemaa el-Fna, Rue Bab Agnaou and in Gueliz. For everyday cash, an ATM usually gives the best rate; exchange desks are useful for the foreign notes you brought.
In more detail
Before you hand anything over, compare the buy and sell rates on the board and ask whether there is a commission. Count your dirhams at the counter before you leave, and keep the receipt. That receipt matters later if you want to change leftover dirhams back.
Never change money on the street. It is illegal, and it is a common set-up for short-changing: notes are counted fast, folded, and some go missing. A licensed desk takes a few minutes longer and removes the risk.
The airport has exchange desks too. They are handy when you land, but rates there tend to be weaker, so change only what you need for the first day and do the rest in the city or at an ATM.
Good to know
- Keep at least one exchange receipt until you fly home.
- Exchange in daylight and put the cash away before you step outside.