If you want to change the currency from an already existing quote you can do it by following steps:
Go to the Magento backend and view the desired quote.
When viewing the quote you can press 'Edit Products' or 'Create order'
You are redirected to the create order view. From there you need to untick the 'Custom Price' box from all of the products and press 'Update Items and Qty's'. After that, you can use the 'Order Currency' to change the currency.
Note: When you untick the Custom Price box, you will lose your custom price and default product prices are applied.
You can now press the 'Update Quotation' button
The quote currency is now changed.