Before you make any changes to the currency, make sure it is a valid address or it will not change. Firstly, go into Store details and change the address of your store and the location where it is located and then press update.
Setup > Store details
Once the store address and country has been updated successfully, the currency should change itself on the back office, however if it does not change, go to
Setup > POS > Localisation > Operating Currency > Update
*Please note, the currency will not change if you do not change the location of your store address, it should automatically change the currency once the location of the store has been changed to another specific country.