At the Business Small and higher versions of Booking Calendar are supported several payment systems integrations.
1) Each payment system support own list of currencies.
So you can change and check the list of the currency at the Booking > Settings > Payment page at the each section of payment system integrations.
2) Also at the Booking > Settings > Payment > General page you can select the currency that is showing in the Booking admin panel and at front-end side (in hints) of the Booking Calendar plugin.
3) In the Booking Calendar Business Medium or higher versions at the Booking > Settings General page in Calendar section, if you have activated this option "Showing cost in date cell", then you can select currency symbol that will be showing in the calendar day cells.