Where to change USD in Accra?

there's plenty of places to change it, but you'll have to ask around for the best one. Ask your driver/taxi driver to take you to a few places and just see which one offers the best rate.

Suggest that you don't start the journey from one of the big hotels though or you'll never get a decent rate and you'll pay 3x as much as you need to for the taxi!

As Jolaan says there are many places. Away from hotels and the Accra Mall, where you will get a very poor rate, they seem to offer rates which do not vary very much. It is probably not worth the cost of a taxi going too far to search. But if you are in the centre of Accra and near the main branch of Barclays Bank, turn down the short street at the side of the building going towards the main Post Office. On the right side of the street there is forex which has always given me a better rate for Sterling than anywhere else. This will also probably be true for USD.

Hi
The have foreign exchange shops all over Accra and other big cities in Ghana. You will find private money exchange boys along major streets and markets in the cities. You can different rates depending where and who change your money.

i will suggest you go the Forex Bureau to change your money and they can be located almost everywhere in Accra and the major cities of Ghana.when you on your way to the heritage restaurant at Osu you will get forex bureau and change your money.remember not to carry alot of money on you because you may never know who is watching.
enjoy your stay in Ghana