In order to configure Multi-Currency in Magento 2 please go to Admin Panel > Stores > Configuration > General > Currency Setup.

Magento 2 currency setup,currency options

Currency Options

To set up the Currency Options expand the section you see below.

1. Set the Base Currency the prices in the 'Admin Panel' are set in.

2. In order to choose the currency which will be shown to the customer by default, set the Default Display Currency. To be able to display currency based on customer Geo location use Auto Currency Switcher Extension.

3. The Allowed Currencies enables you to choose currencies that will be available to a customer on a Storefront.

Magento 2 Currency Options, Base Currency,Default Display Currency,Allowed Currencies

Once you have finished with the Currency Configurations press the Save Config button.

Magento 2 Currency Setup Configuration

Currency Rates

In order to set up Currency Rates go to Admin Panel > Stores > Currency Rates. 

1. This section enables you to configure the multi-currencies rates manually.

2. Otherwise, you can manage to set them up in Import Service choosing the service you want the rates to be automatically imported from.

Magento 2 Currency Rates,Import Service

After finishing don't forget to save changes pressing Save Currency Rates button and then flush Magento cache.

Magento2 Save Currency Rates