Answer: To change default currency from existing currencies means you need to set the new default currency rate to 1. According to the new currency rate of the default currency, the rest of the currencies will change accordingly.
For example, if the CAD currency rate is set to 1, the other two currencies USD and Euro will change in terms of CAD currency rate. In that case, the USD rate will be 0.75 and the Euro rate will be 0.64.