The Base Currency is the currency that's used as the default currency for all new accounts and categories.
To change the Base Currency of your data file, navigate to Tools > Currencies.
Highlight the desired currency name.
Then click the “Set Base Currency” button (at the bottom right of the window).
This will change the base currency of your data file.
It will change the default currency that's used for any new accounts or categories you create.
However, it will not change the currencies of any existing accounts and/or categories.