From the settings page, click on the ‘Default currency’ button:
This will bring up a list of currencies from which you can select one to be set as the default currency for your account:
On the home page your total balance will be displayed in this currency, although you can click on the total balance button and switch between displaying the funds you hold in specific currencies.
The currency which you have set as your default currency will be automatically preselected when sending/requesting funds to/from another Bottlepay user. (The currency in any transaction can be changed easily by clicking the downward facing arrow next to the transaction amount)