Thank you for your question. Since you are running PrestaShop, you would have to custom code a solution, or use an existing module such as "Currency as a dropdown," or "Location & Currency Detection," which tries to detect the currency. I have not tried these specifically, but I found them by performing an online search. You will have to find the module that meets your specific needs.
If you have any further questions, feel free to post them below.
thnx ,the only thing the drop down currency option box is all black in my homepage and not visible how can i make them visible (like the drop down box is black and except for the chosen currency which is white the rest are black) can i change it and make it more visible to clients?