How do I change the default currency displayed for my client?

The currency is updated on a per client basis. You must update the currency symbol used for each client only when the default "$" is inappropriate.

You can change the currency symbol to whatever you like. First open up the client you would like to change. Click on their name at the top right got to Settings > General

In that configuration page click on Reporting and then enter your desired currency symbol in the highlighted field.

After the change click "Save"