General Exchange Rate Default Master record Customer

We can define our own exchange rate types and use them instead of the defaulted types, 'M', 'B' and 'G'. How can we overwrite default types in SD?   

@[email protected] | Updated 31 Jan. 2019

A: Exchange rate gets copied from the customer master record. Exchange rate types are to be maintained for the customer in the sales screen of the customer master record. Shipping