What is a base currency? How to change a base currency?

By default, your base currency will be predefined by the system based on your company location.
If your predefined currency is wrong – you may change it after registration in Settings ➝ Company Profile ➝ Your Base Currency field.
Your base currency can only be changed once and it cannot be changed at all after you have raised your first invoice/estimate/expense/product/project.

Note: You are also able to change base currency if you reset your account data (more here).