Dashboard works according to the order of the currency list in your currency application.

If you change the order of the currency list, you will be able to accomplish your purpose.

Go to Currency application.

1)Tap "Add" in the upper-right corner of the screen.

2)Tap "Edit" in the upper-right corner of the screen to edit currency list.

3)Tap right-end of each currency and move up or down to change the order.

The first one will be the base currency for your dashboard.